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Try to get to know the function of each part. The lens is ___
A.
is 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.
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.
in the front of a camera to allow in light and to focus the image onto a viewing surface.