What are the pros and cons of non-extending zoom lenses?

Question

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..

Answer 1

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).

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

You will want to have a non-extending zoom lens when using anamorphic adapters which often need to be tied with a lens support (like Lanparte lens support); not only to support them but to avoid them from turning when using focus, which would skew the image on an anamorphic.

Answer 6

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.