The basic principle of operation of reflex cameras is a mirror that tilts when the photograph is shot and that defines the path that the light can enter the camera. Under normal conditions, the mirror in front of the shutter is inclined at an angle of 45º to deflect the light to a pentaprism and from there to the viewfinder.
When the picture is taken the mirror is quickly moved to a horizontal position to allow the light to pass to the sensor. When this occurs, the lens diaphragm is closed and the shutter is opened to allow the sensor to be exposed to light. The shutter then closes, the diaphragm reopens, and the mirror returns to its initial position. The result is usually a high quality photo that is displayed on the screen of the camera.