Updated: Jan 28, 2021
How does your camera know what exposure to make?
All cameras have what is called a 'light meter' built into them. When you press halfway down on the shutter the camera takes a 'meter reading' of the light that is reflected back to it. So if you are pointing it toward a bright subject which reflects MORE light you will get a different reading than if you point it toward a darker subject that reflects LESS light. Depending on the exposure mode you are shooting in, the aperture, ISO and shutter speed are determined by what the light meter tells you. (See related posts.)
Most cameras have at least three 'metering modes' which can be used for different scenarios to fine tune the exposure.
Metering Modes-This is how the camera meters the available light: Matrix or Evaluative-Camera evaluates the entire scene and come up with an average by reading 100% of the frame. Good for evenly lit scenes. Center-Weight Metering-Camera evaluates the center of the frame and disregards the edges. Good for a subject that is either much lighter or darker than its background and is in the center of the frame. It evaluates about 60-80% of the frame. Also helpful if there are bright or dark spots in the image. Good for portraits and macro photography. Spot Metering-By default the camera will meter only about 4% of the center of the frame in this mode. In the single point focus area mode, wherever the point of focus is that is where the camera will meter the scene. Good for photographing the moon and stage performances .
Metering modes are covered extensively in DSLR Photography ll