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Abbe Number |
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A numerical value indicating the dispersion of optical glass, using the Greek symbol v. Also called the optical constant. The Abbe number is determined by the following formula using the index of refraction for three Fraunhofer's lines: F (blue), d
(yellow) and c (red).
Abbe number = sqrt(d) = nd ∙ 1/nF − nc
An optical glass characteristic distribution chart is a graph using the Abbe number as the horizontal axis and the d line index of refraction as the vertical axis.
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Aberration |
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The image formed by an ideal photographic lens would have the following characteristics:
- A point would be formed as a point.
- A plane (such as a wall) perpendicular to the optical axis would be formed as a plane.
- The image formed by the lens would have the same shape as the subject.
Also, from the standpoint of image expression, a lens should exhibit true color reproduction. If only light rays entering the lens close to the optical axis are used and the light is monochromatic (one specific wavelength), it is possible to realize
virtually ideal lens performance. With real photographic lenses, however, where a large aperture is used to obtain sufficient brightness and the lens must converge light not only from near the optical axis but from all areas of the image, it is extremely
difficult to satisfy the above-mentioned ideal conditions due to the existence of the following obstructive factors: - Since most lenses are constructed solely of lens elements with spherical surfaces, light rays from a single subject point are not formed in the image as a perfect point. (A problem unavoidable with spherical surfaces.)
- The focal point position differs for different types (i.e., different wavelengths) of light.
- There are many requirements related to changes in angle of view (especially with wide-angle, zoom and telephoto lenses).
The general term used to describe the difference between an ideal image and the actual image affected by the above factors is "aberration." Thus, to design a high-performance lens, aberration must be extremely small, with the ultimate objective being
to obtain an image as close as possible to the ideal image. Aberration can be broadly divided into two classifications: chromatic aberrations, which occur due to differences in wavelength, and monochromatic aberrations, which occur even for a single
wavelength.
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Achromat, Achromatic lens |
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A lens which corrects chromatic aberration for two wavelengths of light. When referring to a photographic lens, the two corrected wavelengths are in the blue-violet range and yellow range.
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AF Stop feature |
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Another feature unique to Canon's four Image Stabilized super-telephoto lenses. Four buttons appear on the outer barrel near the front of these lenses; pushing any one will temporarily lock AF if the camera is in the AI Servo AF mode. Custom Functions on
many newer EOS bodies allow these buttons to assume a variety of additional functions.
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Air lens |
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The air spaces between the glass lens elements making up a photographic lens can be thought of as lenses made of glass having the same index of refraction as air (1.0). An air space designed from the beginning with this concept in mind is called an air
lens. Since the refraction of an air lens is opposite that of a glass lens, a convex shape acts as a concave lens and a concave shape acts as a convex lens. This principle was first propounded in 1898 by a man named Emil von Hoegh working for the German
company Goerz.
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Angle of view |
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The area of a scene, expressed as an angle, which can be reproduced by the lens as a sharp image. The nominal diagonal angle of view is defined as the angle formed by imaginary lines connecting the lens second principal point with both ends of the image
diagonal (43.2mm). Lens data for EF lenses generally includes the horizontal (36mm) angle of view and vertical (24mm) angle of view in addition to the diagonal angle of view.
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Angular aperture |
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The angle between the subject point on the optical axis and the diameter of the entrance pupil, or the angle between the image point on the optical axis and the diameter of the exit pupil.
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Aperture / effective aperture |
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The aperture of a lens is related to the diameter of the group of light rays passing through lens and determines the brightness of the subject image formed on the focal plane. The optical aperture (also called the effective aperture) differs from the real
aperture of the lens in that it depends on the diameter of the group of light rays passing through the lens rather than the actual lens diameter.
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Aperture ratio |
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A value used to express image brightness, calculated by dividing the lens effective aperture (D) by its focal length (f). Since the value calculated from D/f is almost always a small decimal value less than 1 and therefore difficult to use practically, it
is common to express the aperture ratio on the lens barrel as the ratio of the effective aperture to the focal length, with the effective aperture set equal to 1. (For example, the EF 85mm f/1.2L lens barrel is imprinted with 1:1.2, indicating that the
focal length is 1.2 times the effective aperture when the effective aperture is equal to 1.) The brightness of an image produced by a lens is proportional to the square of the aperture ratio. In general, lens brightness is expressed as an F number, which
is the inverse of the aperture ratio (f/D).
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Apochromat, apochromatic lens |
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A lens which corrects chromatic aberration for three wavelengths of light, with aberration reduced to a large degree particularly in the secondary spectrum. EF super-telephoto lenses are examples of apochromatic lenses.
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Aspherical lens |
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Photographic lenses are generally constructed of several single lens elements, all of which, unless otherwise specified, have spherical surfaces. Because all surfaces are spherical, it becomes especially difficult to correct spherical aberration in
large-aperture lenses and distortion in super-wide-angle lenses. A special lens element with a surface curved with the ideal shape to correct these aberrations, i.e., a lens having a free-curved surface which is not spherical, is called an aspherical
lens. The theory and usefulness of aspherical lenses have been known since the early days of lens making, but due to the extreme difficulty of actually processing and accurately measuring aspherical surfaces, practical aspherical lens manufacturing
methods were not realized until fairly recently. The first SLR photographic lens to incorporate an aspherical lens was Canon's FD 55mm f/1.2AL released in March 1971. (Leica offered the 50mm f/1.2 Noctilux lens with aspherical surfaces for its rangefinder
cameras many years before 1971.)
Due to revolutionary advances in production technology since that time, Canon's current EF lens group makes abundant use of various aspherical lens types such as ground and polished glass aspherical lens elements, ultra-precision glass molded (GMo)
aspherical lens elements, composite aspherical lens elements and replica aspherical lens elements.
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Chromatic aberration |
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When white light (light containing many colors uniformly mixed so that the eye does not sense any particular color and thus perceives the light as white) such as sunlight is passed through a prism, a rainbow spectrum can be observed. This phenomenon
occurs because the prism's index of refraction (and rate of dispersion) varies depending on the wavelength (short wavelengths are more strongly refracted than long wavelengths). While most visible in a prism, this phenomenon also occurs in photographic
lenses, and since it occurs at different wavelengths is called chromatic aberration.There are two types of chromatic aberration: "axial chromatic aberration," where the focal point position on the optical axis varies according to the wavelength, and
"chromatic difference of magnification," where the image magnification in peripheral areas varies according to the wavelength. In actual photographs, axial chromatic aberration appears as color blur or flare, and chromatic difference of magnification
appears as color fringing (where edges show color along their borders). Chromatic aberration in a photographic lens is corrected by combining different types of optical glass having different refraction and dispersion characteristics. Since the effect of
chromatic aberration increases at longer focal lengths, precise chromatic aberration correction is particularly important in super-telephoto lenses for good image sharpness. Although there is a limit to the degree of correction possible with optical
glass, significant performance improvements can be achieved using man-made crystal such as fluorite or UD glass. Axial chromatic aberration is also sometimes referred to as "longitudinal chromatic aberration" (since it occurs longitudinally with respect
to the optical axis), and chromatic difference of magnification can be referred to as "lateral chromatic aberration" (since it occurs laterally with respect to the optical axis).
Note: While chromatic aberration is most noticeable when using color film, it affects black-and-white images as well, appearing as a reduction in sharpness.
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Circle of confusion |
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Since all lenses contain a certain amount of spherical aberration and astigmatism, they cannot perfectly converge rays from a subject point to form a true image point (i.e., an infinitely small dot with zero area). In other words, images are formed from a
composite of dots (not points) having a certain area, or size. Since the image becomes less sharp as the size of these dots increases, the dots are called circles of confusion. Thus, one way of indicating the quality of a lens is by the smallest dot it
can form, or its minimum circle of confusion. The maximum allowable dot size in an image is called the permissible circle of confusion.
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Circular aperture |
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Certain Canon lenses feature a new Circular Aperture diaphragm unit, which uses curved aperture blades to provide for a more rounded opening as the lens is stopped down. It's especially effective at rendering out of focus background highlights as natural
rounded shapes. In lenses such as the EF 70-200mm f/2.8L IS lens, the lens opening is virtually circular from f/2.8 to f/5.6. These lenses retain all the benefits previously available with Canon's Electromagnetic Diaphragm smooth and consistent stop-down
operation (even at up to 10fps with the EOS-1v), near-silent aperture control, and total absence of mechanical levers or switches in the lens mount.
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Circular polarizing filter |
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A circular polarizing filter is functionally the same as a linear polarizing filter as it only passes light vibrating in a certain direction. However, the light passing through a circular polarizing filter differs from light passing through a linear
polarizing filter in that the vibrational locus rotates in a spiral pattern as it propagates. Thus, the effect of the filter does not interfere with the effect of half-mirrors: allowing normal operation of TTL-AE and AF functions. When using a polarizing
filter with an EOS camera, be sure to always use a circular polarizing filter. The effectiveness of a circular polarizing filter in eliminating reflected light is the same as that of a linear polarizing filter.
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Coating |
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When light enters and exits a lens, approximately 5% of the light is reflected back at each lens-air boundary due to the difference in index of refraction. This not only reduces the amount of light passing through the lens but can also lead to repeating
reflections which can cause unwanted flare or ghost images. To prevent this reflection, lenses are processed with a special coating. Basically this is carried out using vacuum vapor deposition to coat the lens with a thin film having a thickness l/4 the
wavelength of the light to be affected, with the film made of a substance (such as magnesium fluoride) which has an index of refraction of n, where n is the index of refraction of the lens glass. Instead of a single coating affecting only a single
wavelength, however, EF lenses feature a superior multi layer coating (multiple layers of vapor deposited film reducing the reflection rate to 0.2-0.3%) which effectively prevents reflections of all wavelengths in the visible light range. Lens coating is
carried out not only to prevent reflections, however. By coating the various lens elements with appropriate substances having different properties, coating plays an important role in providing the overall lens system with optimum color balance
characteristics.
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Coma, Comatic aberration |
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Coma, or comatic aberration, is a phenomenon visible in the periphery of an image produced by a lens which has been corrected for spherical aberration, and causes light rays entering the edge of the lens at an angle to converge in the form of a comet
instead of the desired point, hence the name. The comet shape is oriented radially with the tail pointing either toward or away from the center of the image. The resulting blur near the edges of the image is called comatic flare. Coma, which can occur
even in lenses which correctly reproduce a point as a point on the optical axis, is caused by a difference in refraction between light rays from an off-axis point passing through the edge of the lens and the principal light ray from the same point passing
through the lens center. Coma increases as the angle of the principal ray increases, and causes a decrease in contrast near the edges of the image. A certain degree of improvement is possible by stopping down the lens. Coma can also cause blurred areas of
an image to flare, resulting in an unpleasing effect. The elimination of both spherical aberration and coma for a subject at a certain shooting distance is called aplanatism, and a lens corrected as such is called an aplanat.
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Contrast |
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The degree of distinction between areas of different brightness levels in a photograph, i.e., the difference in brightness between light and dark areas. For example, when the reproduction ratio between white and black is clear, contrast is said to be
high, and when unclear, contrast is said to be low. In general, quality lenses producing high quality images have both high resolution and high contrast.
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Cos4law |
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States that light fall-off in peripheral areas of the image increases as the angle of view increases, even if the lens is completely free of vignetting. The peripheral image is formed by groups of light rays entering the lens at a certain angle with
respect to the optical axis, and the amount of light fall-off is proportional to the cosine of that angle raised to the fourth power. As this is a law of physics, it cannot be avoided. However, with wide-angle lenses having a large angle of view,
decreases in peripheral illumination can be prevented by increasing the lens aperture efficiency (ratio of the area of the on-axis entrance pupil to the area of the off-axis entrance pupil).
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Curvature of field |
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Curvature of field is a phenomenon which causes the image formation plane to become curved like the inside of a shallow bowl, preventing the lens from producing a flat image of a flat subject. When the center of the image is in focus, the periphery is out
of focus, and when the periphery is in focus, the center is out of focus. The degree of curvature of field is largely affected by the method used for correcting astigmatism. Since the image plane falls between the sagittal and meridional image surfaces,
good correction of astigmatism results in small curvature of field. Since curvature of field cannot be improved very much by stopping down the lens, lens designers reduce it as much as possible using various methods such as changing the shapes of the
various single lens elements making up the lens and changing the position of the aperture. In doing this, one necessary condition that must be satisfied to simultaneously correct astigmatism and curvature of field is Petzvals Condition (1843). Petzvals
Condition states that a lens element is good if a result of zero is obtained when the inverse of the product of the index of refraction and focal length of that lens element is added to the total number of lens elements making up the lens. This sum is
called Petzvals Sum.
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Depth of field |
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The area in front of and behind a focused subject in which the photographed image appears sharp. In other words, the depth of sharpness to the front of sharpness to the front and rear of the subject where
image blur in the film plane falls within the limits of the permissible circle of confusion. Depth of field varies according to the lens' focal length, aperture value and shooting distance, so if these values are known, a rough estimate of the depth of
field can be calculated using the following formulas:
Front depth of field = d ∙ F ∙ a² / (f² + d ∙ F ∙ a) |
Rear depth of field = d ∙ F ∙ a² / (f² − d ∙ F ∙ a)
f: focal length
F: F number
d: minimum circle of confusion diameter
a: subject distance (distance from 1st principal point to subject)
If the hyperfocal distance is known, the following formulas can also be used:
Near Point limiting = (Hyperfocal distance X shooting distance) / (Hyperfocal distance + shooting distance)
Far Point limiting = (Hyperfocal distance X shooting distance) / (Hyperfocal distance - shooting distance)
(Shooting distance: Distance from film plane to subject)
In general photography, depth of field is characterized by the following attributes: - Depth of field is deep at short focal lengths, shallow at long focal lengths.
- Depth of field is deep at small apertures, shallow at large apertures.
- Depth of field is deep at far shooting distances, shallow at close shooting distances.
- Front depth of field is shallower than rear depth of field.
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Depth of focus |
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The area in front of and behind the focal plane in which the image can be photographed as a sharp image. Depth of focus is the same on both sides of the image plane (film plane) and can be determined by multiplying the minimum circle of confusion by the F
number, regardless of the lens focal length. With modern autofocus SLR cameras, focusing is performed by detecting the state of focus in the image plane (film plane) using a sensor which is both optically equivalent (1:1 magnification) and positioned out
of the film plane, and automatically controlling the lens to bring the subject image within the depth of focus area.
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Diffractive Optics |
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Diffractive Optics, a revolutionary new lens optical technology that permits super-telephoto lenses that are significantly shorter and lighter than previously possible, while simultaneously improving optical performance by reducing chromatic aberrations
and even spherical aberrations.
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Diopter |
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The degree to which the light ray bundles leaving the viewfinder converge or disperse. The standard diopter of all EOS cameras is set at 1 dpt. This setting is designed to allow the finder image to appear to be seen from a distance of 1m. Thus, if a
person cannot see the viewfinder image clearly, the person should attach to the camera's eyepiece a dioptric adjustment lens having a power which, when added to the viewfinder's standard diopter, makes it possible to easily see an object at one meter. The
numerical values printed on EOS dioptric adjustment lenses indicate the total diopter obtained when the dioptric adjustment lens is attached to the camera.
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Dispersion |
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A phenomenon whereby the optical properties of a medium vary according to the wavelength of light passing through the medium. When light enters a lens or prism, the dispersion characteristics of the lens or prism cause the index of refraction to vary
depending on the wavelength thus dispersing the light. This is also sometimes referred to as color dispersion.
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EMD (Electromagnetic Diaphragm) |
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Designed for use with the digital data transfer of the EOS system made possible by the fully electronic mount, every EF lens incorporates an EMD that electronically controls aperture diameter. The EMD is a diaphragm drive control actuator comprised of a
deformation stepping motor and diaphragm blade unit. Features include the following. Because the system is digitally controlled, its level of precision is far higher than that of mechanical linkage systems. The small rotor blades help deliver excellent
start/stop response and control. Elimination of linkage shock from mechanical levers makes the system extremely quiet. The fully electronic mount system makes it possible to close down the aperture and confirm the setting and depth of field at the touch
of a button. The EMD mechanism delivers superior durability and reliability. Its diaphragm control components are integrated into a single compact unit. And, the electronic control system allows a high degree of freedom in designing unit layout.
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Extension amount |
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With a lens that moves the entire optical system backward and forward when focusing, the amount of lens movement necessary to focus a subject at a limited distance from the infinity focus position.
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Extraordinary partial dispersion |
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The human eye can sense monochromatic light wavelengths within the range of 400nm (purple) to 700nm (red). Within this range, the difference in index of refraction between two different wavelengths is called partial dispersion. Most ordinary optical
materials have similar partial dispersion characteristics. However, partial dispersion characteristics differ for some glass materials, such as glass that exhibits larger partial dispersion at short wavelengths, FK glass which features a small index of
refraction and low dispersion characteristics, fluorite, and glass that exhibits larger partial dispersion at long wavelengths. These types of glass are classified as having extraordinary partial dispersion characteristics. Glass with this property is
used in apochromatic lenses to compensate chromatic aberration.
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Eyesight, Visual acuity |
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The ability of the eye to distinguish details of an object's shape. Expressed as a numerical value which indicates the inverse of the minimum visual angle at which the eye can clearly distinguish two points or lines, i.e. the resolution of the eye in
reference to a resolution of 1'. (Ratio with a resolution of 1' assumed as 1.)
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Far-sightedness |
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The eye condition in which the image of an infinitely distant point is formed to the retina when the eye is in the accommodation rest state.
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Five aberrations of Seidel |
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In 1856, a German named Seidel determined through analysis the existence of five lens aberations which occur with monochromatic (single wavelength) light. These are called the five aberrations of Seidel.
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Flange back |
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Distance from the camera's lens mount reference surface to the focal plane (film plane). In the EOS system, flange back is set at 44.00 mm on all cameras. Flange back is also referred to as flange-focal distance.
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Flare |
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Light reflected from lens surfaces, the inside of the lens barrel and the inner walls of the camera's mirror box can reach the film and fog part or all of the image area, degrading image sharpness. These harmful reflections are called flare. Although
flare can be reduced to a large extent by coating the lens surfaces and using anti-reflection measures in the lens barrel and camera, flare cannot be completely eliminated for all subject conditions. It is therefore desirable to use an appropriate lens
hood whenever possible. The term "flare" is also used when referring to the effects of blurring and halo caused by spherical and comatic aberration.
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Floating system |
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General photographic lenses are designed to achieve an optimum balance of aberration compensation at only one commonly-used shooting distance. Thus, although aberrations are well compensated at the reference shooting distance, aberrations increase at
other shooting distances (especially at close shooting distances) and cause image degradation. To prevent this from happening, a floating system is used which varies the interval between certain lens elements in accordance with the extension amount. This
method is also referred to as a close-distance aberration compensation mechanism.
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Fluorite |
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Fluorite has extremely low indexes of refraction and dispersion compared to optical glass and features special partial dispersion characteristics (extraordinary partial dispersion), enabling virtually ideal correction of chromatic aberrations when
combined with optical glass. This fact has long been known, and in 1880 natural fluorite was already in practical use in the apochromatic objective lenses of microscopes. However, since natural fluorite exists only in small pieces, it cannot be used
practically in photographic lenses. In answer to this problem, Canon in 1968 succeeded in establishing production technology for manufacturing large artificial crystals. Thus opening the door for fluorite use in photographic lenses.
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Focal length |
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When parallel light rays enter the lens parallel to the optical axis, the distance along the optical axis from the lens' second principal point (rear principal point) to the focal point is called the focal length. In simpler terms, the focal length of a
lens is the distance along the optical axis from the lens' second principal point to the film plane when the lens is focused at infinity.
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Focal point, Focus |
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When light rays enter a convex lens parallel to the optical axis, an ideal lens will converge all the light rays to a single point from which the rays again fan out in a cone shape. This point at which all rays converge is called the focal point. A
familiar example of this is when a magnifying glass is used to focus the rays of the sun to a small circle on a piece of paper or other surface; the point at which the circle is smallest is the focal point. In optical terminology, a focal point is further
classified as being the rear or image-side focal point if it is the point at which light rays from the subject converge on the film plane side of the lens. It is the front or object-side focal point if it is the point at which light rays entering the lens
parallel to the optical axis from the film plane side converge on the object side of the lens.
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Focus Preset |
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A feature on the Image Stabilized super-telephoto EF lenses. The photographer can focus upon a subject and memorize that focus setting, and later return instantly to it with a brief turn of the metal "playback" ring on the lens' barrel.
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Fraunhofer's lines |
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Absorption lines discovered in 1814 by a German physicist named Fraunhofer (1787-1826), comprising the absorption spectrum present in the continuous spectrum of light emitted from the sun created by the effect of gases in the sun's and earth's
atmospheres. Since each line is located at a fixed wavelength, the lines are used for reference in regard to the color (wavelength) characteristics of optical glass. The index of refraction of optical glass is measured based on nine wavelengths selected
from among Fraunhofer's lines. In lens design, calculations for correcting chromatic aberrations are also based on these wavelengths.
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Fresnel lens |
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A type of converging lens, formed by finely dividing the convex surface of a flat convex lens into many concentric circle-shaped ring lenses and combining them to extremely reduce the thickness of the lens while retaining its function as convex lens. In
an SLR, to efficiently direct peripheral diffused light to the eyepiece, the side opposite the matte surface of the focusing screen is formed as a fresnel lens with a 0.05mm pitch. Fresnel lenses are also commonly used in flash units, indicated by the
concentric circular lines visible on the white diffusion screen covering the flash tube. The projection lens used to project light from a lighthouse is an example of a giant fresnel lens.
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Front group linear extension |
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The rear group remains fixed and only the front group moves straight backward and forward during focusing. Examples of front group linear extension lenses include the EF 50mm f/2.5 Compact Macro and the EF 85mm f/1.2L USM.
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Front group rotational extension |
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The lens barrel section holding the front lens group rotates to move the front group backward and forward during focusing. This type of focusing is used only in zoom lenses and is not found in single focal length lenses. Representative examples of lenses
using this method are the EF 35-80mm f/4-5.6 USM and EF 100-300mm f/5.6L. Since the filter attachment ring and hood rotate with the lens during focusing, care must be taken when shooting through a glass window to make sure the end of the lens does not
contact the glass.
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Full-time manual focusing |
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A system that allows the photographer to turn the lens' manual focusing ring and instantly override autofocus while the lens' AF/MF switch is still in the autofocus mode. More than half of Canon's EF lenses with Ultrasonic Motors have this feature.
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Fully electronic mount system |
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Development of the EOS system began with Canon's own "body range-finding and in-lens motor drive system" and "fully electronic mount system", technologies that were developed in 1985 to quickly respond to the trend towards full-fledged autofocusing SLR
cameras. The EOS system centers on the camera body and consists of various components including Canon's full line of EF lenses, Speedlite flash units, and interchangeable backs. The three main features of the EOS system are as follows. - Multi-processor system control
A high-speed processor in the camera body interfaces with processors in the lens and the flash units, (for high-speed data processing, calculation and communications), to carry out high level systems control. - Multi-actuator system
The ideal actuator for each drive unit is located near the drive unit to form a multi-actuator system that realizes high-level automation, high efficiency, and high performance. - Fully electronic interface
All data transfer between the camera body, lens, flash and interchangeable back is handled electronically. This not only increases the functionality of the current system, but also creates a network ready to accept future system developments.
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Ghost image |
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A type of flare occurring when the sun or other strong light source is included in the scene and a complex series of reflections among the lens surfaces causes a clearly defined reflection to appear in the image in a position symmetrically opposite the
light source. This phenomenon is differentiated from flare by the term "ghosting" due to its ghost-like appearance. Ghost images caused by surface reflections in front of the aperture have the same shape as the aperture a ghost image caused by reflections
behind the aperture appears as an out-of-focus area of light fogging. Since ghost images can also be caused by strong light sources outside the picture area, use of a hood or other shading device is recommended for blocking undesired light. Whether or not
ghosting will actually occur when the picture is taken can be verified beforehand by looking through the viewfinder and using the camera's depth-of-field check function to close down the lens to the actual aperture to be used during exposure.
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Hyperfocal distance |
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Using the depth of field principle, as a lens is gradually focused to farther subject distances, a point will eventually be reached where the far limit of the rear depth of field will be equivalent to "infinity." The
shooting distance at this point, i.e., the closest shooting distance at which "infinity" falls within the depth of field, is called the hyperfocal distance. The hyperfocal distance can be determined as follows: Hyperfocal distance = f² / (d ∙ F) f: focal length
F: F number
d: minimum circle of confusion diameter Thus, by presetting the lens to the hyperfocal distance, the depth of field will extend from a distance equal to half the hyperfocal distance to infinity. This method is useful for presetting a large depth of field and
taking snapshots without having to worry about adjusting the lens focus, especially when using a wide-angle lens. (For example, when the EF 24mm is set to f/11and the shooting distance is set to the hyperfocal distance of approximately 1.5m/4.9ft, all
subjects within a range of approximately 70cm/2.3ft from the camera to infinity will be in focus.)
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Image circle |
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The diameter of the sharp image circle formed by a lens. Interchangeable lenses for 35mm format cameras must have an image circle at least as large as the diagonal of the 24 x 36mm image area, and EF lenses generally have an image circle of about 43.2mm.
TS-E lenses, however, are designed with a larger image circle of 58.6mm to cover the lens tilt and shift movements.
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Image distance |
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The distance from the lens' rear principal point to the film plane when the lens is focused on a subject at a certain distance.
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Image magnification |
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The ratio (length ratio) between the actual subject size and the size of the image reproduced on film. A macro lens with a magnification indication of 1:1 can reproduce an image on film the same size as the original subject (actual size). Magnification is
generally expressed as a proportional value indicating the size of the image compared to the actual subject. (For example, a magnification of 1:4 is expressed as 0.25x.)
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Image stabilizer |
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A superb new technology that allows the lens to sense movement from "shake" or vibrations, and instantly apply an optical correction by moving a group of lens elements. The improvement in steadiness can be seen even in the viewfinder, and most users find
they can shoot hand-held or on a monopod at shutter speeds about two stops slower than previously possible and consistently get sharp images.
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Index of refraction |
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A numerical value indicating the degree of refraction of a medium, expressed by the formula n=sin i/sin r is a constant which is unrelated to the light ray's angle of incidence and indicates the refractive index of the refracting medium with respect to
the medium from which the light impinges. For general optical glass, "n" usually indicates the index of refraction of the glass with respect to air.
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Inner focusing |
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Focusing is performed by moving one or more lens groups positioned between the front lens group and the diaphragm.
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Linear polarizing filter |
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A filter which only passes light vibrating in a certain direction. Since the vibrational locus of the light allowed to pass through the filter is linear in nature, the filter is called a linear polarizing filter. This type of filter eliminates reflections
from glass and water the same way as a circular polarizing filter, but it cannot be used effectively with most auto exposure and autofocus cameras as it will cause exposure errors in AE cameras equipped with TTL metering systems using half-mirrors, and
will cause focusing errors in AF cameras incorporating AF rangefinding systems using half-mirrors.
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Macro lenses |
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Macro lenses are essential for shooting close-ups of flowers, insects and other small items at life-size magnification or larger. Quality optical characteristics, sharp definition and true color fidelity combine to capture the appeal of your subject with
bold realism.
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Mechanical distance |
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The distance from the front edge of the lens barrel to the film plane.
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Micro USM |
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The micro USM is an advanced motor developed as a "multi-purpose miniature ultrasonic motor", and its features are as follows.
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MTF charts - How to read |
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MTF charts (short for Modulation Transfer Function) provide a graph analyzing a lens ability to resolve sharp details in very fine sets of parallel lines, and a lens contrast or ability to provide a sharp transfer between light and dark areas in sets of
thicker parallel lines. Fine repeating line sets are created parallel to a diagonal line running from corner to corner of the 35mm frame, directly through the exact center of the image area. These are called sagittal lines, sometimes designated S on
Canons MTF charts. At a 90° angle to these, additional sets of repeating lines are drawn, called Meridional (or M) line sets. Repeating extremely fine short parallel lines spaced at 30 lines per millimeter measure the lens ability to record fine details,
or its resolution. Even more important in the eyes of many optical designers is the lens contrast capability, which is measured with thicker sets of parallel repeating lines drawn at 10 lines per millimeter. At first glance, it would appear that any good
lens would record lines running parallel to a diagonal drawn across the film with the same accuracy as lines drawn perpendicular to them. However, in real-world testing, this is often not the case. Especially in the Meridional direction, faithful
reproduction of fine line sets becomes increasingly difficult as you move away from the center of the image toward one of the corners. And its a fact that almost all lenses produce sharper results in general near the center of the frame than at the outer
edges. MTF charts display the lens performance from center to corner. Running along the charts horizontal axis, labeled 0 to over 20, is the distance from the dead center (0) of a 35mm image along a diagonal line to the corner of the frame, which is about
21.5mm away. On the charts vertical axis is a scale representing the degree of accuracy with which the fine and coarse line sets are reproduced, in both the sagittal (parallel to the diagonal of the film format) and meridonal directions. Solid lines on
the MTF charts indicate the performance of sagittal lines (parallel to the diagonal of the film), dashed lines are for the perpendicular meridional test target lines. In theory, a perfect lens would produce nothing but straight horizontal lines across the
very top of an MTF chart, indicating 100% accurate reproduction from the center of the picture (toward the left of the chart) to its outermost corners (at the right side of the chart). Of course, no such thing as a perfect lens exists from any SLR
manufacturer, so MTF charts typically show lines that tend to curve downward as they move left to right (tracking the lens performance from center to corner of the frame). Canons MTF charts give results at two apertures: wide-open, and stopped down to
f/8, with the lens set to infinity focus. While MTF charts dont include many factors that can be important when selecting a lens (size, cost, handling, closest focusing distances, AF speed, linear distortion, evenness of illumination, and of course
features like Image Stabilization which may produce superior real-world results), they can indicate to the knowledgeable reviewer some of the optical characteristics they can expect from a particular lens.
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Near-sightedness |
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The eye condition in which the image of an infinitely distant point is formed in front of the retina when the eye is in the accommodation rest state.
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Normal vision, emmetropia |
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The eye condition in which the image of an infinitely distant point is formed on the retina when the eye is in the accommodation rest state.
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Numerical aperture (NA) |
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A value used to express the brightness or resolution of a lens' optical system. The numerical aperture, usually indicated as NA, is a numerical value calculated from the formula nsinØ, where 2Ø is the angle (angular aperture) at which an object point on
the optical axis enters the entrance pupil and n is the index of reflection of the medium in which the object exists. Although not often used with photographic lenses, the NA value is commonly imprinted on the objective lenses of microscopes, where it is
used more as an indication of resolution than of brightness. A useful relationship to know is that the NA value is equal to half the inverse of the F number. For example, F 1.0 = NA 0.5, F 1.4 = NA 0.357, F2 = NA 0.25, and so on.
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Optical axis |
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A straight line connecting the center points of the spherical surfaces on each side of a lens. In other words, the optical axis is a hypothetical center line connecting the center of curvature of each lens surface. In photographic lenses comprised of
several lens elements, it is of utmost importance for the optical axis of each lens element to be perfectly aligned with the optical axes of all other lens elements. Particularly in zoom lenses, which are constructed of several lens groups that move in a
complex manner, extremely precise construction is necessary to maintain proper optical axis alignment.
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Overall linear extension |
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The entire lens optical system moves straight backward and forward when focusing is carried out. Representative examples of lenses using this type of focusing include the EF 50mm f/1.8 II and TS-E 90mm f/2.8.
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Parallel pencil of rays |
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A group of light rays traveling parallel to the optical axis from an infinitely far point. When these rays pass through a lens, they converge in the shape of a cone to form a point image within the film plane.
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Paraxial ray |
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A light ray which passes close to the optical axis and is inclined at a very small angle with respect to the optical axis. The point at which paraxial rays converge is called the paraxial focal point. Since the image formed by a monochromatic paraxial ray
is in principle free of aberrations, the paraxial ray is an important factor in understanding the basic operation of lens systems.
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Peripheral illumination |
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The brightness of a lens is determined by the F number, but this value only indicates the brightness at the optical axis position, i.e., at the center of the image. The brightness (image surface illuminance) at the edge of the image is called peripheral
illumination and is expressed as a percent (%) of the amount of illumination at the image center. Peripheral illumination is affected by lens vignetting and the cos4 (cosine 4) law and is inevitably lower than the center of the image.
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Polarised light |
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Since light is a type of electromagnetic wave, it can be thought of as uniformly vibrating in all directions in a plane perpendicular to the direction of propagation. This type of light is called natural light (or natural polarized light). If the
direction of vibration of natural light becomes polarized for some reason, that light is called polarized light. When natural light is reflected from the surface of glass or water, for example, the reflected light vibrates in one direction only and is
completely polarized. Also, on a sunny day the light from the area of the sky at a 90º angle from the sun becomes polarized due to the effect of air molecules and particles in the atmosphere. The half-mirrors used in autofocus SLR cameras also cause light
polarization.
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Principal point (Nodal point) |
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The focal length of a thin, double-convex, single-element lens is the distance along the optical axis from the center of the lens to its focal point. This center point of the lens is called the principal point. However, since actual photographic lenses
consist of combinations of several convex and concave lens elements, it is not visually apparent where the center of the lens might be. The principal point of a multi-element lens is therefore defined as the point on the optical axis at a distance equal
to the focal length measured back toward the lens from the focal point. The principal point measured from the front focal point is called the front principal point, and the principal point measured from the rear focal point is called the rear principal
point. The distance between these two principal points is called the principal point interval.
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Principal ray |
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A light ray which enters the lens at an angle at a point other than the optical axis point and passes through the center of the diaphragm opening. Principal light rays are the fundamental light rays used for image exposure at all diaphragm openings from
maximum aperture to minimum aperture.
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Rear focusing |
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Focusing is accomplished by moving one or more lens elements positioned internally, behind the lens diaphragm assembly. By moving internal elements, less weight is required to be moved, so focusing can be faster and more responsive. Furthermore, the front
of the lens does not move during focusing ideal for photographers who use filters.
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Reduction in overall lens length |
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To reduce the length of a telephoto lens, it is necessary to increase the mutual power of the convex-concave groupings. Fluorites low index of refraction makes it possible to achieve significant reduction in lens length while maintaining high image
quality.
Although the extraordinary optical properties of fluorite were discovered in the 19th century and lens designers have long desired to use it, naturally formed pieces of fluorite large enough for use in lens production are extremely difficult to find.
Deciding to solve this problem, Canon took up the challenge of developing synthetic crystals, bringing practical fluorite production technology on-line by the late 1960s.
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Reflection |
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Reflection differs from reflection in that it is a phenomenon which causes a portion of the light striking the surface of glass or other medium to break off and propagate in an entirely new direction. The direction of propagation is the same regardless of
wavelength. When light enters and leaves a lens which does not have an anti-reflection coating, approximately 5% of the light is reflected at the glass-air boundary. The amount of light direction of propagation. The two elements of a light wave which can
actually be detected by the human eye are the wavelength and amplitude. Differences in wavelength are sensed as differences in color (within the visible light range) and differences in amplitude are sensed as differences in brightness (light intensity).
The third element which cannot be detected by the human eye is the direction of vibration within the plane perpendicular to the light waves direction of propagation.
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Resolution |
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The resolution of a lens indicates the capacity of reproduction of a subject point of the lens. The resolution of the final photograph depends on three factors: the resolution of the lens, the resolution of the film, and the resolution of the printing
paper. Resolution is evaluated by photographing, at a specified magnification, a chart containing groups of black and white stripes that gradually decrease in narrowness, then using a microscope to observe the negative image at a magnification of 50x. It
is common to hear resolution expressed as a numerical value such as 50 lines or 100 lines. This value indicates the number of lines per millimeter of the smallest black and white line pattern which can be clearly recorded on the film. To test the
resolution of a lens alone, a method is used in which a fine resolution chart is positioned in the location corresponding to the film plane and projected through the test lens onto a screen. The numerical value used for expressing resolving power is only
an indication of the degree of resolution possible, and does not indicate resolution clarity or contrast.
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Shading |
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A phenomenon where light entering the lens is partially blocked by an obstruction such as the end of a lens hood or the frame of a filter, causing the corners of the image to darken or the overall image to lighten. Shading is the general term used for the
case where the image is degraded by some type of obstacle that blocks light rays which should actually reach the image.
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Shooting distance (camera distance) |
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The distance from the film plane (focal plane) to the subject. The position of the film plane is indicated on the top of most cameras by a special symbol like the one below.
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Spherical aberration |
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This aberration exists to some degree in all lenses constructed entirely of spherical elements. Spherical aberration causes parallel light rays passing through the edge of a lens to converge at a focal point closer to the lens than light rays passing
through the center of the lens. (The amount of focal point shift along the optical axis is called longitudinal spherical aberration.) The degree of spherical aberration tends to be larger in large-aperture lenses. A point image affected by spherical
aberration is sharply formed by light rays near the optical axis but is affected by flare from the peripheral light rays (this flare is also called halo, and its radius is called lateral spherical aberration). As a result, spherical aberration affects the
entire image area from the center to the edges, and produces a soft, low-contrast image which looks as if covered with a thin veil. Correction of spherical aberration in spherical lenses is very difficult. Although commonly carried out by coming two
lenses-one convex and one concave-based on light rays with a certain height of incidence (distance from the optical axis), there is a limit to the degree of correction possible using spherical lenses, so some aberration always remains. This remaining
aberration can be largely eliminated by stopping down the diaphragm to cut the amount of peripheral light. With large aperture lenses at full aperture, the only effective way to thoroughly compensate spherical aberration is to use an aspherical lens
element.
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Stop/diaphragm/aperture |
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The opening which adjusts the diameter of the group of light rays passing through the lens. In interchangeable lenses used with single lens reflex cameras, this mechanism is usually constructed as an iris diaphragm consisting of several blades which can
be moved to continuously vary the opening diameter. With conventional SLR camera lenses, the aperture is adjusted by turning an aperture ring on the lens barrel. With modern camera lenses, however, aperture adjustment is commonly controlled by operating
an electronic dial on the camera body.
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Subject distance |
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The distance from the lens front principal point to the subject.
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Super Spectra coating |
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All EF lenses are coated in accordance with Canons own standards, which are even more strict than the CCI tolerances set by the ISO (International Standards Organization), and the variety of single and multilayer coatings used are selected to optimally
match the refraction of the lens to which it is being applied. Named Super Spectra coating by Canon, this process features a high permeation rate, ultraviolet ray filtering, highly durable surface hardness and features and stable characteristics. The
superior imaging characteristics realized by these exacting coating procedures includes sharp, vivid images with high contrast, uniform color balance throughout the EF lens lineup, and true color reproduction that does not change over years of use.
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Super UD lenses |
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The high cost of synthetic fluorite crystal production makes fluorite lenses extremely expensive. One answer was found in the latter half of the 1970s with the appearance of UD (ultra low dispersion) glass that could provide characteristics similar to
fluorite but at a lower cost. While the indexes of refraction and dispersion of UD glass do not equal that of fluorite, they are significantly lower than those of other types of optical glass. Moreover, UD glass does display partial dispersion
characteristics similar to fluorite. The selection of the proper lens element combination in consideration of the intended focal length and other factors can provide close to the same effect as fluorite, (two UD lens elements are equivalent to one
fluorite element). Another breakthrough was made in 1993 when Super UD glass was introduced as a new material that achieves almost the same performance as fluorite while achieving a new balance of greater cost reduction and even higher quality.
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Superior quality across the total image area |
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To achieve a high level of sharpness both at the center and out to the edges of an image when shooting with a telephoto lens, it is desirable for the index of refraction of the front convex lens element to be as small as possible. Accordingly, the use of
fluorite with its low index of refraction effectively improves image quality over the total image area.
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Symmetrical type lens |
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In this type of lens, the lens group behind the diaphragm has nearly the same configuration and shape as the lens group in front of the diaphragm. Symmetrical lenses are further classified into various types such as the
Gauss type, triplet type, Tessar type, Topogon type and orthometer type. Of these, the Gauss type and its derivations is the most typical configuration used today because - its symmetrical design allows well-balanced correction of all types of aberrations, and
- a comparatively long back focus can be achieved.
The Canon 50mm f/1.8 released back in 1951 succeeded in eliminating the comatic aberration which was the sole weak point of Gauss type lenses of that day, and thus became famous as a historical landmark lens due to the
remarkable improvement in performance it afforded. Canon still uses a Gauss type construction in current lenses such as the EF 50mm f/1.8 II, EF 50mm f/1.0L USM, EF 50mm f/1.4 USM and EF 85mm f/1.2L USM. The Tessar and triplet type symmetrical
configurations are commonly used today in compact cameras equipped with single focal length lenses.
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Telephoto ratio |
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The ratio between the overall length of a telephoto lens and its focal length is called the telephoto ratio. Put another way, it is the value of the distance from the apex of the frontmost lens element to the focal plane divided by the focal length. For
telephoto lenses, this value is less than one. For reference, the telephoto ratio of the EF 300mm f/2.8L USM is 0.91 , and that of the EF 600mm f/4L USM is 0.78.
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Telephoto type (teletype) lens |
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With general photographic lenses, the overall length of a lens (the distance from the apex of the frontmost lens element to the focal plane) is longer than its focal length. This is not usually the case with lenses of particularly long focal length,
however, since using a normal lens construction would result in a very large, unwieldy lens. To keep the size of such a lens manageable while still providing a long focal length, a concave (negative) lens assembly is placed behind the main convex
(positive) lens assembly, resulting in a lens which is shorter than its focal length. Lenses of this type are called telephoto lenses. In a telephoto lens, the second principal point is located in front of the frontmost lens element.
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Thorough elimination of the secondary spectrum |
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When a convex fluorite lens is combined with a concave wide-dispersion optical glass lens to correct red and blue wavelengths, the partial dispersion characteristics of the fluorite also effectively compensate the green wavelength as well. This greatly
reduces the presence of secondary spectrum and brings all three wavelengths ~ red, green and blue ~ together at the same focal point to realize virtually ideal chromatic aberration compensation, (apochromatic performance).
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UD glass lenses |
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Lenses made from fluorite are extremely expensive due to the high cost of synthetic fluorite crystal production. UD (ultra low dispersion) glass made an appearance in the latter half of the 1970s, delivering a special optical glass which could provide
characteristics similar to fluorite but at a lower cost and thereby answering another desire of lens designers. While the index of refraction and dispersion of UD glass are not as low as fluorite, they are significantly lower than other types of optical
glass. Moreover, UD glass does display similar partial dispersion characteristics. The selection of the proper lens element combination in consideration of the intended focal length and other factors can provide close to the same effect as fluorite, (two
UD lens elements are equivalent to one fluorite element). Super UD glass was introduced in 1993 as a new material that achieves almost the same performance as fluorite while achieving a new balance of cost reduction and higher quality.
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USM (Ultrasonic Motor) |
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Canon became the first camera maker to apply the use of an advanced USM (Ultrasonic Motor) in 1987 when the EF 300mm f/2.8L USM amazed the world with its silent, super-fast autofocus performance. Then, in 1990, Canon developed the lower cost ring-type USM
could be used in a variety of more affordable lenses. This feat was followed in 1992 by the development of a new type of micro USM that enabled the automation of production. Every day Canon comes closer to realizing the goal of equipping every EF lens
with a USM. Features of the ring-type USM include its ability to easily achieve the low-speed, high-torque characteristics needed to realize direct drive. Large holding torque means the disc brake automatically holds the lens in place when the motor is
stopped. Its construction is extremely simple, operation is virtually noise-free, and it demonstrates excellent start/stop response and control. High efficiency and low power consumption allow the lens to be powered by the cameras battery. The motors ring
shape is optimally suited to lens barrel applications and its low rotation speed is ideal for lens drive purposes. Rotation speed control covers a wide variable range from 0.2 rpm to 80 rpm to realize high-precision, high-speed lens drive control.
Variable-sensitivity electronic manual focus is also available. The broad operating temperature range of -30ºC to +60ºC (-22ºF to 140ºF) ensures stable operation even in severe environments. And, all lens drive control is performed by the microprocessor
housed within the lens.
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Vignetting |
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Light rays entering the lens from the edges of the picture area are partially blocked by the lens frames in front of and behind the diaphragm, preventing all the rays from passing through the effective aperture (diaphragm diameter) and causing light
fall-off in the peripheral areas of the image. This type of vignetting can be eliminated by stopping down the lens.
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What is "light"? |
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According to the dictionary, Light is defined variously as: 1. something that makes things visible or affords illumination; an illuminating agent or source, as the sun, a lamp, or a beacon: 2. electromagnetic radiation to which the organs of sight react,
ranging in wavelength from about 4000 to 7700 angstrom units and is propagated at a speed of about 186,300 miles per second, and including a similar form of radiant energy that does not affect the retina, as ultraviolet or infrared rays; 3. a gleam or
sparkle, as in the eyes; 4. a particular light or illumination in which an object seen takes on a certain appearance; 5. a person who is an illuminating or shining example; luminary; 6. mental or spiritual illumination or enlightenment; 7. the aspect in
which a thing appears or is regarded. The definition most indispensable to the understanding of light as used in photography is 2 above. Types of electromagnetic radiation vary according to wavelength. Starting from the shortest wavelengths,
electromagnetic radiation can be classified into (special form enter) rays, X rays, ultraviolet light rays, visible light rays, infrared light rays, far-infrared light rays, microwave radiation, ultrashort wave radiation (VHF), short-wave radiation,
medium wave radiation (MF) and long wave radiation. In photography, the most utilized wavelengths are in the visible light region (400nm-700nm). Since light is a type of electromagnetic radiation, it can be thought of as a type of wave in the category of
light waves. A light wave can be regarded as an electromagnetic wave in which an electric field and magnetic field vibrate at right angles to each other in a plane perpendicular to the direction of propagation. The two elements of a light wave which can
actually be detected by the human eye are the wavelength and amplitude. Differences in wavelength are sensed as differences in color (within the visible light range) and differences in amplitude are sensed as differences in brightness (light intensity).
The third element which cannot be detected by the human eye is the direction of vibration within the plane perpendicular to the light waves direction of propagation.
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Working distance |
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The distance from the front edge of the lens barrel to the subject. An important factor especially when shooting close-ups and enlargements.
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Zoom lenses |
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A single standard zoom lens does the work of several lenses with fixed focal length. Take advantage of the lens speedy operation to capture the full breadth of a scene with a wide-angle setting, or zoom in instantly to shoot a close-up of some
particularly attractive highlight in telephoto mode. Telephoto zoom lenses bring an added dimension to EOS system performance. Follow moving subjects such as athletes or animals in action, and experience the telephoto lens characteristic shallow depth of
field and compressed effect as you enjoy this added dimension of expressive power.
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