Metering is measuring the light to ensure a proper exposition. All cameras now sold on the market use metering. My first camera, bought 35 years ago had no metering systems.
Metering systems included in cameras today often have multiple modes. Some of these modes might be called Center Weighted, Spot or Matrix metering. In these modes, the metering systems will use larger or smaller part of the frame to calculate the available light.
Whatever the mode you use, your in-camera metering system will guess wrong more often than you would hope for. In-camera metering system is measuring the light reflected from the subject. It then calculates the available light in the scene taking into account that the subject is 18% gray. 18% gray is a shade of gray that hopefully represent the average scene. Should your subject be much darker, like a black bear, or brighter like a bride in its white gown, your metering systems might be off by up to 2 exposure value.
Moreover, you might want to achieve a particular effect that the in-camera metering system is not designed to handle. A picture of silhouette, might need you to underexposed much. A High Key portrait will lead to an overall exposition that is brighter than what the in-camera metering system would recommend.
How to get it right with metering?
- After taking the picture you should look at the monitor on the back of your camera to ensure the metering system got it right;
- Enabling the histogram would also give clues about your exposition after the fact. This could leads you to use exposure compensation and take another shot;
- With experience, looking at a scene, you might decide to use exposure compensation should you know that your subject is much darker or brighter than the usual 18% grey;
- Should you be out for a shoot that span a long period in the same lighting conditions you could take a first shot, make sure your exposition is right and then switch to manual for remaining shots. The subject brightness of future shot will not impact your metering systems then;
- Some experienced photographers shoot in manual mode with much success! It requires years of practices though;
- Using a light meter and measuring the light received by the subject is the greatest way to get it right!
In most situations, photographed scene can stand for some wiggle room. This means that you can adjust the exposition in post-production by 1 stop of light plus or minus without much impact. However, nothing is better than getting it right in camera.
Don’t trust your in-camera metering system. Learn to read the histogram, make sure your camera display the histogram after taking the picture. Enable the clipping warning, you will get it right most of the time. In the included picture, the metering system would have guess it wrong. The histogram got it right though.