This is the attraction of the SLR camera. The major characteristic of this camera is the viewfinder system. Because the image that actually reaches the focal plane is displayed in the viewfinder, the image that will be captured digitally or on film can be checked in advance.
The light rays that pass through the lens are deflected upward by a mirror in front of the lens to show the image in a focusing screen that is equidistant with the focal plane. A pentaprism is then used to correctly orient and project the image before it is seen through the eyepiece. When the shutter button is pressed, the mirror rises up and the shutter opens to expose the film or image sensor. When the shutter closes, the mirror returns to its original position. This makes it possible to accurately frame images without being affected by parallax, a problem with compact cameras, which have different light paths for the shooting lens and the viewfinder.