Try to get to know the function of each part. The shutter is ___
A.
an opaque piece of metal or plastic inside your camera that prevents light from reaching the film or digital sensor. The shutter is opened, or released, by the shutter release button.
B.
the reflective device for the digital single lens camera. It is used to correct the reverse pictures on the focusing screen, so that operator can frame and focus correctly.
C.
equivalent to an ideal film plane. Distance from the mirror to the focusing screen is equal to the distance from the mirror to the film plane.