How does fully automatic exposure (program mode) work?
Your camera has a built in light meter, which it uses the measure the amount of light in the scene at various places. The position of the light meter will vary from model to model. Some popular positions in SLRs are behind the (partially reflective) reflex mirror, or in the pentaprism. On some digital cameras, the main sensor may serve double duty as a light meter. Your camera will determine the light level in the scene and then use an electronic version of an exposure table to pick the appropriate shutter speed and aperture. You may notice that there are multiple combinations that will be suitable for any EV. To the extent that it is possible, cameras will typically try to pick shutter speeds that are compatible with handheld shooting. The more advanced ones will even take the focal length of your lens into account and try to pick higher shutter speeds if needed to reduce the effects of camera movement. One thing you should realize now is that with multiple possible combinations of sh
