A modern digital camera, or DSLR, has a built in light meter system to correctly expose images automatically. These systems often have a range of different metering patterns available. Some settings are more accurate than others, but they all change the patterns by which light levels are measured by the camera in order to choose the correct exposure .
The center-weighted average is the standard metering pattern found in most cameras . Light levels across the whole image are averaged, but preference is given to the central area since it is assumed that is where the main subject is located.
This metering pattern can be expected to give accurate exposures for typical well-lit subjects. However, it will often lead to errors in exposure under uneven lighting situations.
A comparison is made to saved model lighting situations to find the exposure reading. This reduces the risk of exposure error by prevents the reading from being overly influenced by parts of a scene that are very dark or bright, such as the sky.
This exposure pattern segments the frame into four to 16 sections depending on the camera model. More accurate readings are given by a camera with more active zones.
Since emphasis is paced on the center area, this pattern can be fooled by images with an uneven range of tones, or off-centered subjects.
The most sophisticated patterns available today are “intelligent”, multi-pattern systems. They aim to make taking perfectly exposed pictures easy for any photographer. The scene is divided into multiple zones. Each has the light and brightness levels measured and analyzed by a microprocessor to give the best exposure setting.
For situations that could confuse an automatic metering system, an AE (Auto Exposure) lock allows you to take a meter reading and save it. The camera can then be moved to a new position while keeping the same exposure settings. For example, to shoot a landscape you could exclude the sky from the viewfinder by tilting the camera down, taking a meter reading, and using the AE lock to hold the exposure settings while you recompose the shot. This would avoid an underexposure of your subject that the bright sky would otherwise create.
This metering pattern only uses light levels from the central 5-15 percent of the total image. This allows you to take a meter reading from the main subject without letting bright or dark areas of the scene affect the exposure. This is very useful when the subject is either much lighter or much darker than the background. Going in close and taking a reading ensures that you expose the most important parts of the image to their best effect.
Like other automatic exposure patterns, it will not work well for compositions with the metered subject anywhere but the center of the frame.
This pattern is essentially the same as selective metering above, but it measures only a tiny central area of the viewfinder. Usually, only 1-5 percent of the total image area is used. Therefore, you can use it to take meter readings from precise areas of a scene. This makes it one of the most accurate, but hard to use system available.
Multi-spot metering allows the photographer to combine individual spot readings. This average exposure can work well for scenes where it is not clear where to meter from. Combining meters from the lightest and darkest parts of the frame will give an exposure that will provide a good starting point for experimentation.
Any metering pattern is challenged by complex images with a wide range of intensities or if the subject is backlit. Multi-segment metering patterns can analyze the frame and compare the results to a saved database to provide extreme accuracy.
Modern automatic light metering settings can help a beginner or expert photographer quickly take properly exposed shots. But if you don’t have at least a basic understanding of how they work, you will run into much frustration and problems.