With Canon EOS cameras, you have a choice of up to four different metering modes – Evaluative, Spot, Centre-weighted and Partial. Your choice of metering mode will determine how you use exposure compensation.
Evaluative metering breaks the scene down into a number of zones. A reading is taken from each, and the information is analysed by the camera's processor.
If, for example, the central zones are darker than the outer zones, it is likely that the main subject is backlit. Conversely, if the central zones are much brighter than the outer zones, the main subject might be in a spotlight. In both cases, the camera will bias the exposure to the central zones, giving correct exposure to the subject.
The trouble when using exposure compensation with evaluative metering is that you don't know if the metering has already compensated for the conditions. If it has and you dial in exposure compensation, then the exposure will be wrong. Equally, if you assume that the camera has got it right but it hasn't, then you will also get a badly exposed picture. You won't know until you review the shot on the screen on the back of the camera. This means it might take a little trial-and-error to get the result you want.
Centre-weighted metering takes a reading from the whole scene, but it puts more emphasis on the results from the central area. Unlike evaluative metering it makes no attempt to analyse the scene, so you can apply exposure compensation in the knowledge that the camera has not made any adjustments of its own. For this reason, centre-weighted metering is often better than evaluative when you know that some level of exposure compensation is needed.
Unlike evaluative metering, partial metering and spot metering take the reading from just a small area of the scene, and they are much easier to use with exposure compensation. You are in full control of where the camera takes a reading from, and you know that the camera is not making any further adjustments.
In partial and spot metering mode you can also take the reading from a mid-tone area, if there is one (for example, an area of grass), which means that you don't need to apply any exposure compensation.
However, if there is no mid-tone in the frame, you can take a reading from a light or a dark area and then apply exposure compensation. For example, if you take a reading from a white area (not an extreme highlight, but one that still retains some detail), +1.5 or +2 stops of compensation should give similar results to a reading from a mid-tone area with no compensation. Similarly, a partial or spot reading from a dark area (one that still retains detail), together with -1.5 or -2 stops compensation, will also give results similar to a reading from a mid-tone area.
If the area used for the reading is not in the centre of the frame, set the exposure compensation first, then move the camera so that the area is in the centre. Now partially depress the shutter button to take a reading, and use the exposure lock button to hold the reading as you recompose the image. Finally, press the shutter button fully to take a picture.