Backwards Zoom Lenses?

The lens that came with my first DSLR was the Pentax DA 16-45mm. A nice lens but a bit odd: 16mm FL at greatest physical extension, 45mm FL at shortest. Couldn't understand why it should be that way round, but just got on with using the lens and forgot about it. Subsequent acquisitions were all the 'normal' way round: longest FL at greatest extension, shortest FL at shortest extension. Until getting the Q-02 5-15mm recently. That is like the 16-45mm: shortest FL at greatest extension and vice versa. Which got me wondering again. So do any of you clever people have any idea:
- How backwards zooming works optically.
- Why Pentax (or other designer) would use a backwards design for a zoom lenses.
- Whether there any other fairly common zoom lenses that are backwards.
- Whether the DA 16-45mm and Q-02 are related optically.

Steve

all i know is the 16-45 was highly recommended and still is .

It all depends on the optical design.

For example, most front elements are convex, but a few are concave. I concur that the 16-45mm is an excellent lens.

Working that way round can cause shadowing issues using the onboard camera flash at wider angles.

I think most of my zoom lenses are shortest at intermediate focal lengths, usually extending slightly for the shortest FL & more for the longest setting. This is certainly the case for at least half a dozen around the 'normal' focal length.

I'm sure it's a consideration that comes quite a long way down the list of priorities in lens design, in fact probably so far down it's not even on the page.

There are a few lenses with unusual designs like that. The M 75-150mm f/4, while it doesn't get any longer or shorter when zooming as the zoom ring moves along the barrel, is at 150mm with the zoom ring closest to the camera. This seems to me to be good design as it adds stability when the lens is at it's longer focal length, which is where it's most needed.

I remember being surprised to find that, while the lens lengthens when focusing closer to the subject, the rear element of the DA 35mm limited stays in the same place always and doesn't move with the rest of the lens elements.

I remember seeing a lens test for the M 75-150mm f/4 in LPMM and it showed the lens to be sharper at the 150mm end than the 75mm. Normally it's the other way round.

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