What are the functions of "aperture priority" and "shutter priority" of SLR cameras, and when are they used?

Aperture priority: the photographer decides the aperture size, while the shutter is set by the camera according to the aperture size and the light conditions at that time. Because the depth of field is closely related to the aperture size, this mode is convenient to control the depth of field of photos.

Shutter priority: the shutter is decided by the photographer. The camera sets the aperture according to the shutter speed and light conditions. This mode is used to shoot fast-moving objects, or occasions that need to show "movement", such as gurgling streams, or night scenes that need long exposure.

Digital SLR camera is a single lens reflex digital camera, referred to as SLR (single lens reflex). This technology is to install a mirror at a 45 angle in front of the film plane, and there are frosted glass and pentaprism eyepiece above the mirror in turn. The pentaprism reflects the real image light many times to change the optical path, and sends the image to the eyepiece, so that the image seen in the observation window is always consistent with the image on the film, and the shooting range is basically the same as the actual shooting range. The unique design of this prism enables the photographer to directly observe the image passing through the lens from the viewfinder.