Ever wonder how your camera picks the right exposure (not too bright or too dark) for most of your shots? That is because most cameras have built in light meters. The purpose of the meter is to measure the amount of light being reflected back into the camera from a particular subject or from a whole scene. The camera then calculates the best shutter speed and aperture opening combination to yield a properly exposed image. (or you set it yourself if you have the option of making manual exposure settings).