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Apart from the obvious benefit of not changing the center of gravity, what are the technical pros and cons of zoom lenses that do not change "physical length" when changing the focal length?

For example, are they more or less prone to mechanical failure? More or less prone to attract dust in dirty environments? etc..

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I don't think there's anything inherently different about the two designs apart from the change in length of the lens. Two particular lenses might show differences in durability, etc, but a general rule seems almost impossible to come by here. –  ex-ms Sep 2 '10 at 21:33
    
Are we talking non-extending zoom, or non-extending focus? I know of lenses that do not extend during focus, but I think that zoom still requires extension to one degree or another. Even if it is in a "non-extending tube", such as the Canon EF 16-35mm L II, there is still extension regarding zoom. Focus, however, is often all-internal with enclosed moving groups. I guess I'm just confused on the real subject matter here. –  jrista Sep 10 '10 at 19:44

5 Answers 5

up vote 9 down vote accepted

The benefits for a non-extending zoom are:

  • Can typically be fully moisture and dust sealed.
  • Do not suffer from zoom-creep, where the lens slowly starts to extend when gravity is pulling on it. Some lenses (like the Canon 100-400) have a way to increase tension to minimize this, but this is the exception to the design.
  • No change to physical size (this is kind of obvious).
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Extending during zooming:

Pros:

  • Usually smaller when unextended and able to take up less space in storage.
  • Cheaper and easier to manufacture?

Cons:

  • Sucks in dust but more importantly moisture, can't be weather sealed.
  • Possibility of zoom-creep (focal length changing when hung around your neck).
  • Front element may rotate slightly (not nearly as much as below)

Extending during focusing:

Pros:

  • Maintains focal length at closer focusing distances. This is a big one and a common complaint of Internal-Focusing lenses.

Cons:

  • Front element will rotate a lot, requiring adjustment of filters and eliminating the possibility of a petal hood.
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Good answer, hope you don't mind my edit. I think the rotating front element is nowadays rather an exception than a rule. –  Karel Sep 10 '10 at 17:28

They don't suck in dust or moisture when they are zoomed. If you are using them for macro work, they don't bang into the subject when zoomed. Typically, IF zooms don't rotate on zoom either, so they are easy to use with graduated filters and polarisers.

Much of this is secondary though. Zooms intended for pros have many useful features, and non-extension is just one of them. Others include being more rugged, being weather sealed (not the same thing) and often, higher quality.

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Zoom lenses that change length are definitely more prone to sucking in dust, since they have some expanding space inside, at that's going to have to be filled with air that used to be outside.

Other issue is that fixed-length lens is less likely to zoom by itself when you have camera hanging on your neck.

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One thing to consider is that on some zoom lenses, the front element will rotate when focusing. I've seen this on some cheaper lenses. Not necessarily a problem, but you will need to readjust any mounted filters. This is not an issue on more expensive lenses, which tend to not rotate when focusing.

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