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>>50234 See the little knobbly thing sticking out the front of the lens? That's a mirror. What this lens does is the light goes in around that, bounces off a mirror in the back, bounces off the mirror in the front, and finally goes into the camera. It's a quick and easy way to cram a lot of millimeters into a small space. The downside, however, is that you're stuck at a fixed aperture (I.e., it's always f/5.6. Never f/4, never f/8, always f/5.6) and you lose some optical quality in all the bouncing, and your bokeh is (arguably) ugly as sin.
This optical design is used a lot for telescopes, but not as often for cameras because of the aforementioned aperture and quality issues. However, they're way cheaper and smaller than the big-ass equivalents with better optical quality and more than one available aperture.
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