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1. In SLR single lens Reflex  cameras, the image from the lens is redirected by 90
 deg change, it its direction.  It is projected on to the small view finder screen.

2. another reason could be to avoid Chromatic and spherical aberration due to lenses

3. a good reason to use prisms of different shapes, sizes in cameras is to create a lot of visual effects in the picture captured.  Many movies utilize such methods.
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