Backwards Zoom Lenses?


Lubbyman

Link Posted 13/05/2019 - 10:41
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

redbusa99

Link Posted 13/05/2019 - 12:08
all i know is the 16-45 was highly recommended and still is .
K3 II and the odd lens or 2

Flickr

johnriley

Link Posted 13/05/2019 - 13:36
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.
Best regards, John

JAK

Link Posted 13/05/2019 - 19:05
Working that way round can cause shadowing issues using the onboard camera flash at wider angles.
John K
Last Edited by JAK on 13/05/2019 - 19:10

petrochemist

Link Posted 14/05/2019 - 00:22
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.
Mike
.
Pentax:K5ii, K7, K100D, DA18-55, DA10-17, DA55-300, DA50-200, F100-300, F50, DA35 AL, 4* M50, 2* M135, Helicoid extension, Tak 300 f4 (& 6 film bodies)
3rd Party: Bigmos (Sigma 150-500mm OS HSM),2* 28mm, 100mm macro, 28-200 zoom, 35-80 zoom, 80-200 zoom, 80-210 zoom, 300mm M42, 600 mirror, 1000-4000 scope, 50mm M42, enlarger lenses, Sony & micro 4/3 cameras with various PK mounts, Zenit E...
Far to many tele-converters, adapters, project parts & extension tubes etc.

.[size=11:].Flickr WPF Panoramio

Jonathan-Mac

Link Posted 14/05/2019 - 08:19
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.
Pentax hybrid user - Digital K3 & K200D, film 645 and 35mm SLR and Pentax (&other) lenses adapted to Fuji X digital
Fan of DA limited and old manual lenses

Algernon

Link Posted 14/05/2019 - 09:01
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.

--
Half Man... Half Pentax ... Half Cucumber

Pentax K-1 + K-5 and some other stuff

Algi
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