Modern digital cameras are very good at selecting the correct exposure metering automatically. However most DSLR and high end Point-and-shoots have multiple metering modes which many people never use. Switching from the default mode can have a dramatic effect on the end result.
Digital cameras use something called a reflective meter. This means they measure light reflected from objects in the scene. Dedicated handheld meters can measure reflective and ambient light (light falling on the object rather than reflected).
Many digital cameras contain a built-in database of images with associated exposure settings. When the camera measures exposure in metering mode it compares the scene with this database. The results allow the camera to work out the best settings for a correct exposure.
For most cameras, the exposure meter will only see in black and white and to obtain the correct exposure it uses what we call 18% grey as a reference. This is a mid-point grey that has been worked out from the average light levels of thousands of different scenes.
Unfortunately the light reflected from your scene may be darker or brighter than 18% grey. This could lead to incorrect exposure. The built-in image database goes some way to resolving this issue, however it’s not infallible. This is where selecting the correct metering mode will help.
This is the default for most DSLRs. The camera breaks the scene down into zones. Each zone is independently metered and the average exposure is used for the entire scene. For most shots this form of metering is ideal.
As the name suggests centre weighted metering measures predominantly from the centre of the image. The rest of the scene is also measured but when calculating the exposure more weight is given to the centre measurements.
This metering method works well where the main subject is in the centre of the frame, such as a person in a field or on a road.
When in this mode the camera only measures a very small area in the centre of the view finder, usually 5%. Some DSLRs allow this value to be adjusted. The main use for this mode is perfect exposure on your subject, for example preventing a backlit subject being too dark.
When in spot metering mode, you direct the exposure point at your subject, lock the exposure, usually with a half press of the shutter or with the exposure lock button if your camera has one, and then recompose the image. Remember, to return to centre weighted or matrix modes when finished as spot metering in general will cause wild fluctuations in your exposure if you leave it on by mistake.
Some cameras provide more metering options; many can now employ 3D metering. This uses the distance information from the lens to determine what the subject should be and exposing accordingly