The built in meter in your camera is known as a reflective light metering system,
This means that the meter evaluates the elements that you are pointing the camera
at and tries to give you the correct exposure or iris setting when you shoot.

Now basically all reflective meters are trying to find middle gray. The principal
being that if we get middle gray or middle of the exposure scale right, then the
highlights and the shadows will fall where they should.

Let’s just imagine that the light meter in your camera could talk and you pointed
your camera (while The meter is on automatic mode) toward a subject that was
overall very bright with Lots of white highlights.

The meter would say…”Ooops! this subject is far too bright for middle gray so I had better close down the iris of the lens to let less light in.”, which means you under expose the subject.

If you did the same thing and pointed the camera at something with a lot of dark
tones in it then the meter would say… “Ooops! This subject is to dark, we had
better open up the iris”. In both cases you would wind up with the wrong exposure.

Now if the whole world was middle gray, you wouldn’t have any problems with this.

The entire above situation is why professional cinematographers and still photographers for that matter often use an incident light meter. This is the one with the little ping pong type ball on it that you often see being held in front of the actor’s face.

The principle here is that an incident meter measure the light falling on the subject without regard to whether It is black or white and there-bye can give a more accurate and consistent exposure reading from the master shot to the medium shot and to the close up shot when using a single camera.

I often demonstrate this principle in my cinematography class and turn the students on to getting far better exposure while using a video camera.

I do workshops on this kind of information. Call me if you would like me to put one on in your area.

Cheers, Barry