Marcus Hawkins

The meter measures a subject’s brightness so that the camera can determine how long the sensor needs to be exposed to record a picture. The problem is that the metering system doesn’t always work flawlessly, and you may end up with pictures that are either too dark or too bright. For more refined results, you can correct these errors using exposure compensation, or dial in the exposure settings – aperture, shutter speed and ISO – manually.

Camera meters are calibrated to what’s called ‘18% grey’. The theory is that a mid-tone grey, halfway between black and white, reflects 18% of the light falling on it. Point your camera at a grey card or a rough mid-tone equivalent, such as a field of grass or a pavement, and the camera will produce a well-exposed result. Obviously, not everything you photograph falls neatly into this mid-tone range. For instance, a swan in a snowy field will reflect much more light, while a black cat in a coal cellar reflects much less.

This is why photos of these subjects can look too dark or too bright: the metering system is trying to bring the overall exposure closer to mid-tone grey. The exposure for the swan will be decreased, so it comes out looking dull and grey, while the exposure for the black cat will be increased, so it comes out looking washed out and grey. There are a range of variables that have an effect on the exposure, including the metering mode being used. ‘pattern’ metering mode takes a range of readings across the entire picture, then calculates the optimum exposure according to the brightness of the scene or subject.

Each camera manufacturer has its own take on this pattern metering mode – Canon calls it Evaluative, while Nikon plumps for Matrix – but they effectively do the same thing. The idea is that you shouldn’t have to tweak the exposure, as the camera has effectively applied its own exposure compensation. That white swan? It should come out closer to white when you use pattern metering.

In reality, though, this metering mode isn’t a magic bullet: low light, excessively bright or dark subjects or ones that are very small in the frame can still throw the metering out, and you may need to apply some exposure correction to get the right result.

Your camera also comes with a centre-weighted metering mode. As the name suggests, this meters the whole scene but gives priority to the centre of the frame. It’s a throwback to the days of film photography and doesn’t make any adjustment to the exposure itself.

Finally, there’s spot-metering. This measures the brightness in a very small part of the frame. This is a great option when you want to lock the exposure on a small subject, or to take a number of readings from across a scene and then calculate the optimum exposure yourself.