When you zoom in with a lens on an SLR why does the lens go in then out?

Question

I guess this is more a question of optics than photography but I just got an SLR with a basic 18-55 lens. I noticed that when going from 18 to 55 or 55 to 18 the lens physically comes back in and then physically goes back out?

What is going on there? I would think that if I am zooming in the lens should be going out 100% of the time but the lens actually goes out and then comes back in.

Answer

The lens is retrofocal at the wide end and telephoto at the long end. A retrofocus lens is referred to as "inverted telephoto" because it is constructed similarly to a telephoto lens with the elements reversed. The effect decreases as you zoom in, until you reach about 35mm, at which the lens begins to extend and eventually becomes a telephoto configuration, where the size of the lens, front element to rear element, is less than the focal length. The lens is neither retrofocal nor telephoto between these positions. This results in the lens being longer at the extremes of the zoom range than at intermediate positions.

For more information on this design, see the Wikipedia articles on Angénieux retrofocus, which discusses the origin of the design for the wide end, and telephoto lens for what happens at the long end. According to the telephoto lens article:

Zoom lenses that are telephotos at one extreme of the zoom range and retrofocus at the other are now common.

This is essentially what is happening with your 18-55mm lens. As far as I am aware, Canon, Nikon, Pentax, and Sony (A-mount, not E-mount) 18-55mm lenses all share this design aspect.

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