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I saw that in Why do some people prefer 50mm to 35mm prime lenses, even for a crop sensor? , in @MattGrum's answer:

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The 50mm is a simpler design compared to the retrofocus 35mm
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What's retrofocus?

24

Fundamentally a telephoto design is used to make a lens body shorter than it's focal length, for practical reasons [what if your 18-300 ultrazoom was actually 30cm long?], a retrofocus design is the opposite, and makes a lens longer  than it's focal length in order to leave space for the mirror on an SLR.

You can tell if your lens is a retrofocus or telephoto by removing both caps and holding it in front of a bright background so you can see the aperture (the hole in through which light passes). For a retrofocus lens this aperture will look bigger from the back than from the front. For a telephoto lens it will look smaller from the back than from the front (the extreme telephotos you see have lens barrels which are much narrower toward the rear so this would have to be the case).

If it's about the same size front and back, then you have a symmetric  lens, such as the 50mm I was referring to. This is a simpler design as it is neither retrofocus or telephoto and this allows them to be made cheaply despite having a large aperture such as f/1.8.

11

Retrofocus lens is a reversed telephoto lens in which the front element is negative, moving the rear nodal plane (from which the focal length is measured) behind the rear of the lens. It is the standard design for wide-angle and fisheye lenses in SLR and cine cameras where a conventional lens would foul the shutter mechanism.

See here for a simple explanation. Also see here for more info.

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