Exposure and Metering
- Wed, 21 Jul 2010
Photographs are made of light, and to be successful the sensor must receive the right amount of it. Too much or too little and the picture will fail to record some parts of the scene properly.
The precise amount required, however, varies according to the subject, the type and intensity of the light available, and the effect required. Bright subjects, for example, reflect more light than dark ones, so require less to make a picture.
Your camera's exposure meter, which measures the brightness of the scene, assumes it contains an equal number of dark and light tones. Its job is to find the average of those tones and then set an exposure that will reproduce that average as a mid-grey.
Most of the time this works fine, but with subjects that are predominantly light or dark (such as a snow scene, or night photography) then your camera is likely to give you a wrong reading and you may have to intervene to override it. Otherwise you could end up with grey snow.
Image: This statue is almost a silhouette, as the meter was influenced by the sky
In this mode your viewfinder is divided into a grid of between six and 51 segments (depending on the camera), each of which takes its own reading to form a pattern of light distribution.
These are then compared with a database to help determine the exposure. It is programmed to recognise (and ignore) bright skies, for example, or know when a main subject is backlit.
In practice this system, which goes by various names (e.g. Matrix, Evaluative) usually works well and should be your default mode for general shooting.
The old-school metering mode, in which the camera takes a single reading from a medium-sized zone around the centre of the frame, then tries to make it mid-grey. It doesn't attempt to interpret what it sees and apply compensation - that's your job.
This mode is popular with some experienced photographers because it's predictable. With practice you'll get to know when the meter will be fooled and can override it accordingly.
This takes a reading from a very small area (between 1° and 5°) so you can set the exposure for a specific subject within the scene and exclude all else. With a portrait, for example, you could take a reading directly from the subject's cheek.
Overriding the meter
If your exposure meter gets things wrong, your camera provides various means to override it.
This button, marked with a plus/minus icon, enables you to increase or decrease the metered exposure by up to three stops either way.
This bias will then be applied to every shot you take until you reset the compensation to zero.
Image: This white interior was turned grey until +1 stop compensation was added
The auto bracketing feature automatically shoots a sequence of images in rapid succession at different exposure increments (pre-chosen by yourself) - thus ensuring that one of them will be correct. This is good for those situations where exposure is critical and you don't want to have to check every shot.
Image: Here the dark background caused the performer to be over-exposed
Manual exposure gives you total control. The camera gives you a meter reading but it's for you to interpret as your experience and judgement decides is required. With manual mode the exposure value will not change between shots even if the light level does, so you need to keep an eye on light levels.
Image: Here the meter was fooled by the window and under-exposed the interior