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

I apologize if the wording of my question isn't that great. I have a Nikon D5100 with the "default" (came with the camera) 18-55mm lens, and I've seen several different types of lens', that all zoom in, then back out, but I'm curious as to whether there's a lens that actually zooms out, rather than in. Sorry for the wordy question, and thanks.

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Adra Elkins
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    When you say zoom in and out, are you referring to the physical barrel of the lens moving in and back out? If so, then it is because the 18-55mm uses a retrofocal design. Other (most) lenses do zoom out. See this: http://photo.stackexchange.com/questions/18451/when-you-zoom-in-with-a-lens-on-an-slr-why-does-the-lens-go-in-then-out – MikeW Jan 28 '13 at 00:05
  • @mikew I don't know if you're right, but thanks for providing that link, I had missed a good question and a great answer. – Francesco Jan 28 '13 at 00:08

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Your question is not completely clear to me. I will try to answer what I have understood.

A zoom lens allows the choice of a focal length in a given range. We call zooming in the action of choosing a longer focal length, since the field of view is reduced to a smaller portion of what the lens is facing. Note that the lens might, or might not, appear to be physically longer: the relevant length is given by the optical path. So a typical lens appears to get longer while zooming, but this is not a rule. For example the Canon EF 24-70mm f/2.8 extends in the "wrong" direction: when it appears to be longer the focal length is shorter and vice versa.

So for a zoom lens (as opposed to a so called prime lens, which cannot zoom and thus has a single, fixed focal length) you can choose a focal length in a range, and call it zooming in or zooming out depending if you're going from the shorter to the longer focal length or vice versa.

Francesco
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Yes, there is. For example, I have a Canon EF 24-70mm, where the front lens element moves out when the focal length goes from 70mm to 24mm.

One advantage with this design is that the lens hood attaches behind the part of the lens that moves, so it actually retracts relative to the front lens when the view angle gest wider, so it works well for the full range, not just at the widest angle.

Guffa
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I believe the effect you are seeing, where the mechanical position of the front of the lens goes inward, then back outward, as you zoom from the shortest focal length to the longest, is due to the way the lens optics have to work in a different way between wide angle and telephoto. A telephoto-only lens (short focal length longer than 50mm or so) would be what you need. A 70mm to 200mm telephone zoom lens would be the obvious example.

Skaperen
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Are you talking about "zoom" when you just grab a camera? Usually zoom lens are more compact when set to widest zoom (18mm in your case) and are easier to store and carry around. If you want to zoom out, then zoom in, just store it at 55mm :-)

Though some bigger zoom lenses suffer from the 'zoom creep' and can be locked at wide angle to avoid it happening.

Dmitriy
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