Exposure meters can measure only how much light comes from a scene. The meter cannot know if that scene is a light scene in dark conditions or a dark scene in bright light. So, the meter really gives an exposure that results in an average or middle-gray tonality for everything, which is not appropriate for every subject.
Camera metering systems have multiple metering points over the image area to give the camera’s built- in metering computer something more to work with than a single (and perhaps misleading) meter reading. Still, certain types of scenes present problems.
An image made up of dark tones over the whole composition will usually be overexposed because the meter wants to make it bright, not “knowing” that the scene is not bright. A photo that has mostly bright tones will typically be underexposed because the meter wants to darken it, not “understanding” that the scene is not darker.
Finally, a scene with a strong contrast between the subject and background often causes the meter to improperly expose the subject. A bright background causes a dark subject to become underexposed, and a dark background can make the subject overexposed.
RAW files can be helpful in difficult metering conditions. With RAW software, you can often bring out detail in the shadows while holding tonalities in the highlights, even when the light is contrasting and hard to meter. That does not mean you can be sloppy with RAW exposures, though. You need to do the best you can for every subject in order to get the best possible detail and color from a scene.
One thing you pay for with
more expensive cameras is more metering points in the scene for the metering system to evaluate. With more points, the system can better find the key tones to favor, while ignoring single-point anomalies of light, such as a bright spot in an overall moderate-toned image.