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Originally Posted by Peterg984
what are these lenses like? they seem very small in length to be 500mm.
cheers
Pete
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Mirror (catadioptric) lenses achieve the short physical length and the long focal length by bouncing the image back and forth off mirrors within the lens. They usually have a fixed aperture of f8 or f11 so you're limited to using them only in the best light. They also typically give a pronounced doughnut effect with out of focus highlights in shots.
Best avoided IMHO, unless you're going to go for a recognized brand, and even then, they're really a cheap alternative to a conventional lens. You can usually fit small neutral density filters over the rear lens element which is the only way (with the fixed aperture) to have any control over exposure.
They're optically not as good as a normal lens, and that one on fleabay looks really cheap and nasty as Joe says.
