Canon’s iFCL (intelligent Focus Colour Luminance) metering system is used on all its current DSLRs. This takes both colour and brightness information into account, as well as the focus setting.
When you’re using Evaluative Metering in One Shot autofocus mode, the exposure setting is heavily biased towards the active focus point.
So, for example, if you’re using multi-point AF and only one of the points locks onto a target within a scene, the light metering will be based on this point alone.
Evaluative metering therefore works in a different way to a conventional ‘multi’ metering mode that takes all parts of the scene into account.
It works well for subjects such as backlit portraits, where you’re using a single AF point on the face of the person you’re photographing, and you want to capture the correct exposure for the subject, regardless of how much the background is overexposed.
In other situations, however, the metering can be thrown if the AF point that achieves autofocus falls on a particularly light or dark part of your scene.
To take full control and get more consistent results in tricky lighting conditions, it often pays to switch to one of the other metering modes available on your Canon DSLR.
Centre-weighted average uses the central region of the scene as the main basis for light metering, but also takes the periphery into account to a lesser degree.
The Partial metering mode is a bit more picky, as it uses only the central region and completely disregards what’s happening around the edges of the scene. Spot metering is the most critical mode, as it uses only a very small point at the centre of the frame and everything else is disregarded – you therefore need to be very careful with your aim when you’re using this mode.
It’s also important to understand the difference that alternative autofocus modes can make to metering. In One Shot autofocus mode, metering will be locked at the moment that autofocus is achieved, for as long as you maintain a light press on the shutter button before finally fully pressing it to take your shot.
In AI Servo autofocus mode, which you’d use to track moving targets, the metering updates continuously as the focus setting adjusts itself. In AI Focus mode, metering remains fixed as long as the target remains stationery.
However, if the target moves and AI Focus switches to tracking mode, metering will be updated automatically.
You can lock the metering system to an initially captured exposure value, regardless of which autofocus mode you’re using, by pressing the AEL (Auto Exposure Lock) button on the back of the camera – it’s labelled with a star.
Overall, you can combine the various different autofocus and light metering modes to work in pretty much any way you want once you understand what each one does, making the most of the great versatility offered by current Canon DSLRs.
Step by step – Choose the right metering mode
This works well for general shooting, especially when you’re using multi-point autofocus, where a number of different autofocus points are likely to lock on to foreground areas with different brightness levels. Light and dark areas in the foreground will then be averaged out to produce a well-balanced exposure.
Where you’re most interested in a particular part of a scene but still want to avoid the sky washing out, or shadows being too dark, use Centre-weighted metering and single-point AF, using the centre autofocus point to focus on the main subject before swivelling the camera to recompose if necessary.
A small area at the centre of the frame is used for Partial metering; all other areas are disregarded. This is a good choice for shots such as backlit portraits. Lightly press and hold the shutter button to achieve autofocus and metering, then recompose if necessary and fully press the shutter button to take your shot.
For tricky, high-contrast scenes, Spot metering is often the best choice. If, for example, you’re photographing a very bright subject against a dark background, or vice-versa, you can take a light reading with pinpoint precision. You may also need to use exposure compensation to fine-tune the results.