How to control exposure

To understand exposure, it can be helpful to think of your camera as a tool for gathering light. Exposure is simply the amount of light we allow in. If we allow too little light in then the picture will be dark, or underexposed. Too much and it may become overly bright, or overexposed.

Ambient light bounces off objects and surfaces in a scene and your lens channels it through the camera and onto the sensor – this is what creates the image. In Auto or Scene mode, Canon cameras are very good at determining the optimum amount of light for a balanced exposure, but we can have greater creative freedom if we take control ourselves.

There are three factors that allow us to control exposure, known as the exposure triangle. First there's the aperture, which is the size of the opening in the lens that allows light through and is measured as an f-number.

Each lens has a range of aperture settings from large to small. Large apertures are expressed as small numbers, such as f/2.8, letting in more light and producing a shallow depth of field (increased blur beyond your point of focus). Small apertures are described by larger numbers, such as f/16, letting in less light and producing large areas in focus from foreground to background.

The second factor in our exposure triangle is shutter speed, which is the length of time we allow light to hit the sensor. Like water passing through a tap, the longer we keep the shutter open, the more light travels through.

A fast shutter speed (sensor open for a short amount of time) can be useful for capturing high-speed action, while a slow shutter speed (sensor open for a long time) can be used for creative motion blur effects, such as light trails or light painting.

The third factor in our exposure triangle is ISO. This lets us adjust how sensitive the sensor is to light. Lower ISOs such as ISO100 result in better quality photos. High ISOs make the sensor more sensitive, so you'll need less light for a correct exposure. But the trade-off here is increased image noise, and at very high ISOs the quality of the image will start to be affected.

As a rule, it's best to use the lowest possible ISO, but modern Canon cameras have class-leading high ISO performance, so you can happily shoot upwards of ISO1600 or more and still capture beautifully detailed photos in low light.

The three key factors in our exposure – aperture, shutter speed and ISO – work in combination, so when you change one factor you need to balance it out with another if you want to maintain the same exposure, or allow the camera to compensate for it. As such, it can be helpful to think of them as the three sides of a triangle. Adjust one of the sides, then either one or both of the other two sides need to adapt too, unless you're specifically trying to increase or decrease the exposure levels.

For example, increasing the shutter speed to freeze fast action may underexpose your image. To compensate you need to increase the aperture or ISO. Similarly, decreasing your aperture to maximise front-to-back focus will require a reduction of either the shutter speed or ISO. Start to think of exposure in this way and you soon realise there are a wide combination of settings that will result in a similarly exposed photo, and you have the creative freedom to choose the right combination for your subject or scene.

Your Canon camera makes it easy by offering several exposure modes that let you shift the exposure triangle one way or another. In Program (P) mode, you control the ISO and the camera selects both the shutter speed and aperture. Use Shutter priority (Tv) mode and you can change the shutter speed while letting your camera look after the overall exposure. Similarly, in Aperture priority (Av) mode you determine the f-number and the camera sets the correct shutter speed. You can select Auto ISO or choose to set the ISO yourself. In Manual (M) mode you can set any combination of aperture, shutter speed and ISO you like.

EOS R System cameras also have a Flexible priority (Fv) mode, which enables you to switch between controlling your aperture, shutter speed, exposure compensation and ISO within the same mode. When you change one setting, the rest automatically adjust to maintain the correct exposure, which makes exposure control much quicker and easier.

Adjusting exposure in this way can be used for creative effect. Darkening your images will deepen colours and give your images a more moody feel, particularly in landscape photography. Increasing your exposure will brighten the shadows and produce a more modern feel.

Exposure bracketing enables you to capture a scene at a number of exposures and choose which one works best afterwards. Choose between ±1 and ±3 stops on the exposure scale and allow the camera to make the necessary adjustments to your settings.

When there is less light we may need to keep the shutter open for longer. This is sometimes a necessity, and at other times a creative choice to allow motion blur within the scene, perhaps to blur the movement of water or clouds. The camera needs to stay perfectly still, so a tripod or stable surface is essential to prevent camera shake. Even pressing the shutter button can cause issues, so it's best to use a remote shutter release using the Canon Wireless Remote Control BR-E1, or the Canon Camera Connect app on your phone to trigger the shutter remotely. The app also allows you to shoot Bulb exposures, which enables you to keep the shutter open for several minutes or more, which is perfect for shooting fireworks or stars at night.

The challenge when shooting long exposures during the day is setting the shutter speed to allow enough movement to occur. Ideally you want shutter speeds of more than a few seconds, so remember to keep your ISO low and aperture small. It also helps to shoot in overcast or shady conditions or at either end of the day when the light is dimmer. Alternatively, invest in a filter that fixes on the end of your lens and prevents some light from entering. A neutral density (ND) filter will work best.

Once you've mastered your exposure, the creative opportunities are endless. Most Canon mirrorless and DSLR cameras feature a multiple exposure mode that lets you blend more than one frame. Inspired by the multiple exposure technique from the days of film in which the same frame of film would be exposed twice or more, this feature lets you create all kinds of stunning effects.

Of the three in-camera blending options, Additive is most like the film technique, as it combines the brightness values so that white areas stay white. When shooting in this mode, look for bold shapes, strong contrast and striking silhouettes.

Next time you take your camera out, try experimenting with the exposure triangle and see for yourself the changes it can make to your photos.

Written by James Paterson