Canon cameras are clever but the exposure metering system can't always select settings that produce exactly the result you want, because it can't know exactly what you're photographing. You might be shooting a light-toned subject in poor light or a dark-toned subject in bright light – it is all much the same to your camera.

In fact, most exposure metering assumes that scenes approximate a mid-tone grey area. This is usually taken as one that has 18% reflectance – that is, the scene reflects about one-fifth of the light falling on it. So if you're photographing a very light-toned subject and the camera detects the bright light it reflects into the lens, the metering system interprets this as a lot of light falling onto a grey scene and reduces the shutter speed and/or aperture to compensate, with the result that your image is underexposed. In the same way, if the scene has lots of dark tones, the camera assumes it is a mid-tone scene in low light and increases the shutter speed and/or aperture, leading to overexposure.

When you're shooting in Manual (M) exposure mode, you can accommodate the subject or scene brightness through the exposure settings that you select. In the fully-automatic or semi-automatic modes like Program (P), Aperture priority (Av) and Shutter priority (Tv) mode, you can adjust the exposure to suit the scene using exposure compensation.

With some EOS cameras, if the ISO (sensitivity) is set to Automatic, you can also use exposure compensation in Manual exposure mode.

The principle of exposure compensation is straightforward. To make the next picture you take darker, use a negative exposure compensation setting. To make your next shot brighter, use a positive exposure compensation setting. However, exposure compensation is set in different ways depending upon the camera model, and the exact process will be explained in the camera's user manual.

In some instances you need to half-press the shutter button to awaken the camera and take an exposure meter reading, then press and hold the +/- Av button while rotating the Electronic Input Dial to set the exposure compensation. By default, rotating the dial to the right sets positive exposure compensation, while rotating it to the left sets a negative value.

In other cases you need to half-press the shutter button to activate the camera and take a meter reading and then use the Quick Control Dial to set the compensation value.

Once you have set exposure compensation on all current EOS cameras, it will stay set until you reset it back to zero again, even if you turn the camera off and back on again. This can catch you out if you forget, so when you've finished taking a series of pictures, remember to reset the compensation to zero to avoid unwanted results later.

By default, the exposure compensation is adjustable in 1/3-stop increments and usually up to +/-5EV, but some EOS cameras allow you to choose between 1/2-stop and 1/3-stop exposure compensation increments via a custom function in the main menu.

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.

There is no foolproof guide to the amount of exposure compensation you need. It will vary with every scene you photograph, and many shots will not need any compensation.

"If it's light, go right" is a handy way to remember which direction you need to go on the exposure compensation scale: if the scene is lighter than a mid-tone, you need to move in the direction of positive compensation to improve the exposure.

As a rough guide, try +2 to +3 stops for scenes with lots of snow or sand, which reflect a lot of light and are likely to cause the camera's metering system to underexpose, and +1 if there is a lot of water behind your subject. Negative exposure compensation is likely to be needed less often, and around –1 will handle most dark backgrounds.

If you're using a mirrorless EOS camera, provided that Exposure simulation is enabled in the menu, the image in the viewfinder and LCD screen will show a preview of the image with the exposure settings applied. This means that you can use the viewfinder image as a guide to how much exposure compensation is required.

How do you know if your images are over- or underexposed? Basically, they look wrong! But this can be a subjective matter. If you are photographing something light or white and it comes out looking grey or murky, the picture is underexposed. Similarly, if you are photographing something dark or black and the image looks grey, it is overexposed.

There is no such thing as a correct exposure. There are good exposures and bad exposures, but ultimately it is down to how you want the image to look. Some people deliberately "underexpose" an image by 1/3 stop to make the colours in the image appear more saturated.

Your camera's histogram display – a graphical representation of the range of tones in a shot – can be an invaluable guide to help you assess an image's exposure and judge whether any highlights or shadows are being clipped, which means that detail is being lost. Enable the Highlight alert on cameras that have this feature to see where clipping might be occurring in the brightest areas of the scene.