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04-24-2010, 09:36 PM   #31
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I'm the OP and like Stillshunter above, I was kind of keeping score and actually walked away with thoughts of the lens possibly being "slightly" varifocal to be honest. I was willing to concede that the lens can lose a little focus when jumping in and out of focal lengths, or at least it's expected behavior for the majority of us holding these lenses.

SMC Pentax-A 35-105mm f/3.5 Macro - How about more data? - Photo.net Pentax Forum
(Andrew Gilchrist):
QuoteQuote:
Most zoom lenses maintain focus reasonably well when adjusting focal length though some will shift a little bit. I think you may be right that the A35-105/3.5 does a pretty good job at this.
I interpreted that as it certainly not being perfect, but at least acceptable. Either way, there's definitely an open question. Maybe someday we can have a line judge call it, (and more interestingly); whether or not the focus is supposed to stay spot on throughout the focal range

04-25-2010, 12:27 AM   #32
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Well we'll have to agree to disagree. I am leaning towards the lens being parfocal. Having said that, my exercise has been purely an academic one, as I do not have one to test. However, having corresponded with someone who owns the lens, and who has tested with focus at 105, then zooming out to 35mm and the lens maintaining a relatively sharp image. So my vote is with the A35-105 being a true zoom - parfocal. If all goes well I should have one of my own to test in a matter of days and will post my findings...though the rebuttal remains that, "oh yeah but yours is busted"....

Lucky you and I aren't line judges then...we'd have one confused umpire

By the way writeb, are you buying one yourself?
04-25-2010, 09:27 AM   #33
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In my experience the lens is parfocal.

The SMC Pentax-A Lenses manual of January, 1983 recommends zooming all the way in to the subject to focus, then zooming out to compose.
04-25-2010, 09:46 AM   #34
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QuoteOriginally posted by monochrome Quote
In my experience the lens is parfocal.

The SMC Pentax-A Lenses manual of January, 1983 recommends zooming all the way in to the subject to focus, then zooming out to compose.
Well there it is.. The definitive answer. Finally somebody actually read the instructions .. I guess I could have done the same thing...



04-25-2010, 01:18 PM   #35
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QuoteOriginally posted by monochrome Quote
In my experience the lens is parfocal.

The SMC Pentax-A Lenses manual of January, 1983 recommends zooming all the way in to the subject to focus, then zooming out to compose.
QuoteOriginally posted by JeffJS Quote
Well there it is.. The definitive answer. Finally somebody actually read the instructions .. I guess I could have done the same thing...

I tried . . . I really tried to resist the temptation to go look it up

I couldn't find a direct statement from Pentax or any other sources whether the lens is par-or vari-focal. but the instructions certainly imply that it is parfocal.

Page 23. The Password is Pentax

BTW - nice equipment list - beats mine all to hell

Last edited by monochrome; 04-25-2010 at 01:55 PM.
04-25-2010, 01:44 PM   #36
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QuoteOriginally posted by JeffJS Quote
Finally somebody actually read the instructions ..
Spoilsport! My excuse is that I don't have instructions (not that I would have actually read them.)
04-25-2010, 06:42 PM   #37
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I would only consider this lens parafocal at certain distances. The minimum focusing distance of this lens changes from 35mm to 105mm.
04-25-2010, 07:16 PM   #38
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QuoteOriginally posted by hangu Quote
I would only consider this lens parafocal at certain distances. The minimum focusing distance of this lens changes from 35mm to 105mm.
Well, that statement doesn't work. Either it is a true zoom or it isn't, not just some of the time.

According to the manual I referenced earlier, Pentax says the minimum focusing distance for this lens is 5 feet. The same measurement is printed on the Distance Scale right on the lens. There is no statement of at which focal length the minimum focusing distance is achieved. Zoom in to the maximum focal length, focus, zoom out to recompose. Focus is maintained.

At least that's what Pentax says,

But what do they know.


Last edited by monochrome; 04-25-2010 at 08:18 PM.
04-25-2010, 07:18 PM   #39
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QuoteOriginally posted by hangu Quote
I would only consider this lens parafocal at certain distances. The minimum focusing distance of this lens changes from 35mm to 105mm.
Huh??

Parfocal lens - Wikipedia, the free encyclopedia

While the minimum focal distance may change with focal length on different (separate) lenses, explain to us how that changes the definition of Parfocal... That is Not the case with mine. I cannot focus any closer with 35mm than I can with 105mm. Demonstrated in the photo below, there is only one focus distance scale. As the distance scale shows, the closest (non macro) focus, is about 1.5 meters, for both 35 and 105.

Here's a photo of the lens to use as a guide.



04-25-2010, 10:33 PM   #40
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I'm all for embedding photos to illustrate a concept pertinent to the topic at hand. This certainly was not one of those moments.

QuoteOriginally posted by JeffJS Quote
Huh??

Parfocal lens - Wikipedia, the free encyclopedia

While the minimum focal distance may change with focal length on different (separate) lenses, explain to us how that changes the definition of Parfocal... That is Not the case with mine. I cannot focus any closer with 35mm than I can with 105mm. Demonstrated in the photo below, there is only one focus distance scale. As the distance scale shows, the closest (non macro) focus, is about 1.5 meters, for both 35 and 105.

Here's a photo of the lens to use as a guide.

Thank you for the link to the Wikipedia entry! I didn't understand parafocal the first two times it was explained in this thread but the third time is always the charm!

All kidding aside, the lens is parafocal (parfocal?) only from the distance of 5 feet to infinity. Since you own this very lens, you must be aware that it has a macro feature that enables you to focus at a distance under 5 feet.

Are we now discounting the macro feature of this lens? Because the macro feature is certainly is NOT parafocal.

QuoteOriginally posted by monochrome Quote
Well, that statement doesn't work. Either it is a true zoom or it isn't, not just some of the time.

According to the manual I referenced earlier, Pentax says the minimum focusing distance for this lens is 5 feet. The same measurement is printed on the Distance Scale right on the lens. There is no statement of at which focal length the minimum focusing distance is achieved. Zoom in to the maximum focal length, focus, zoom out to recompose. Focus is maintained.

At least that's what Pentax says,

But what do they know.
I'm not about to argue with you or the Pentax manual. Based on your past posts, you are certainly more knowledgeable than I am about Pentax cameras and lenses. Therefore, pray tell how you would categorize this lens.
  • At a distance of 5 feet to infinity, it will not lose focus between 35mm and 105mm.
  • Once you shift the focus ring and focus to a distance of under 5 feet, it will quickly lose focus between 35mm and 105mm.
  • The macro section (green line) has no distance scale, simply because the focal distance changes from 35mm to 105mm once it focuses at a distance of under 5 feet.

I agree with you that a parafocal must retain focus along all distances, but this lens does not fall under that stringent criterion, hence why I labeled it as varifocal earlier in the thread.

Last edited by hangu; 04-25-2010 at 10:43 PM.
04-26-2010, 11:59 AM   #41
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QuoteOriginally posted by hangu Quote
I agree with you that a parafocal must retain focus along all distances, but this lens does not fall under that stringent criterion, hence why I labeled it as varifocal earlier in the thread.
Well it is certainly a discussion - not an argument.

This isn't a macro lens; it is a zoom lens that has a close focusing setting which is called "Macro" for marketing purposes. I hae never even set my lens in that mode, much less ever used it.

There should be separate discussions for the normal function and the "close focus" function, since mechancially shifting the rear elements materially changes the optical design of the lens.
04-26-2010, 01:35 PM   #42
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QuoteOriginally posted by monochrome Quote
Well it is certainly a discussion - not an argument.

This isn't a macro lens; it is a zoom lens that has a close focusing setting which is called "Macro" for marketing purposes. I hae never even set my lens in that mode, much less ever used it.

There should be separate discussions for the normal function and the "close focus" function, since mechancially shifting the rear elements materially changes the optical design of the lens.
I see, so the close focusing aspect of the lens does not count in this discussion.
04-26-2010, 02:09 PM   #43
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QuoteOriginally posted by hangu Quote
I see, so the close focusing aspect of the lens does not count in this discussion.
Whatever. You are right. I concede.

When in Normal mode the lens is parafocal.
When intending to focus between 5 feet and infinity, as the lens markings indicate, focus is constant.
When in Macro mode the lens is varifocal.
When intending to focus closer than 5 feet, where there are no lens markings, after mechancially adjusting the lens' optical formula (intentionally changing the lens from a 35~105 zoom to someting else), focus varies.
So since a true zoom lens must be constant-focus always and in all modes to be parafocal this is not a true zoom lens.

Pentax was wrong. I was wrong. You are right.
04-26-2010, 02:29 PM   #44
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QuoteOriginally posted by monochrome Quote
Whatever. You are right. I concede.

When in Normal mode the lens is parafocal.
When intending to focus between 5 feet and infinity, as the lens markings indicate, focus is constant.
When in Macro mode the lens is varifocal.
When intending to focus closer than 5 feet, where there are no lens markings, after mechancially adjusting the lens' optical formula, focus varies.
So since a true zoom lens must be constant-focus always to be parafocal this is not a true zoom lens.

Pentax was wrong. I was wrong. You are right.
I'm going to reiterate what I've stated earlier, I respect what you have to say and I have genuine respect for your knowledge of Pentax lenses. As such, I didn't want to argue with you or a Pentax manual. That wasn't sarcasm or obsequiousness.

My previous response was an attempt to ask you to clear up your comment. Not a sarcastic retort establishing my superiority on this subject (of which, I've freely conceded to you twice in this thread). I have a firm grasp of what a parafocal/varifocal lens is, I just wanted to know how a lens like this would be categorized since it has aspects of both.

I may deserve a snarky or even a rude comment from two other members on this thread, but certainly not from you. I've shown you nothing but the utmost respect for what you have to say.
04-26-2010, 03:04 PM   #45
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When that lens is engaged in macro mode, it cannot zoom. For this reason, whether the lens was para or vari is invalid to that macro mode. You might also consider the macro mode a bonus.
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