Try to get to know the function of each part. The mirror is ___
A.
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.
B.
used to reflect pictures captured by lens to upper pentaprism. (When the photographer presses the shutter, the mirror will bounce and the shutter curtain will bounce too, so that the inside sensor can sense the light and record the picture.)
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.