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05-06-2011, 12:04 PM   #1
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Is this normal for a Pentax-A 35-105mm?

Hello. I have a pentax-A 35-105mm manual lens that I really enjoy using for walks due to its useful focal length. Not to mention being sharp as well. A little bit heavy but I don't mind it at all. I noticed that when I focus on a subject & changes focal length ie. 105mm to 35mm, I have to refocus again. This can be a bit annoying, causing delay especially when you have a moving subject. I don't see this on my other zoom lenses. Is this a normal behavior? Otherwise, great manual lens!

05-06-2011, 12:19 PM   #2
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The ability to maintain focus while zooming depends on the lens design. Some zooms can do it and others can't. If you want to know more look up the terms varifocal and parfocal.
05-06-2011, 01:49 PM   #3
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There's an old thread here in the lens section that goes something like this:

Poster 1: I have this lens and I can zoom from end to end without refocusing.

Poster 2: You are wrong. I have this lens and I have to refocus.

Poster 1: I'm right. And when I bought it, the dealer said that was a feature.

Poster 2: Have you had your vision checked recently? This web site doesn't say anything about that feature, therefore the lens doesn't have it.

Poster 3: Here is the wikipedia entry on parfocal and varifocal. Honestly, is it a big deal to refocus?

Poster 2: Yes, and I have returned this lens, sold my Pentax and bought some hyperexpensive thing for just this feature. My standards are exceptionally high.

Poster 1: I've had 11 National Geographic cover shots with my copy, and I have never changed focus since the day I bought it.

After that, the thread starts to deteriorate.
05-06-2011, 04:32 PM   #4
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I used to own 3 copies. 2 maintained focus from tele to wide, the worst shape one with sign of repair did shift focus.

05-06-2011, 04:47 PM   #5
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I had 3 copies of this lens (all now sold), none of them required re-focusing when zooming.

However, if I focused at 35mm, then zoomed to 105mm, I might need to re-focus. That was because it was easier to see the focusing mistake at 105mm than at 35mm.
05-08-2011, 08:43 AM   #6
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Thanks for the replies. Seems a minor issue given it's good quality. I can live with it
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