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Cooltouch
Tinkerer
Username: Cooltouch
Post Number: 259
Registered: 01-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, April 29, 2013 - 08:41 pm: | 
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I bought this lens from KEH about a year ago, I guess. I noticed an odd problem when I had it out yesterday. It will focus at infinity only at 200mm. At 80mm, about the farthest away it will focus is about five, maybe six feet. And that limit moves slowly out as I zoom out.
A visual inspection of the lens's internals as I zoom in and out don't reveal anything unusual.
I've used this lens a fair amount, but as I sit back and think about it, I guess I was using it only at 200mm. It does make for a very good 200mm f/2.8 lens, but I need to get to the bottom of this so I can use it at other focal lengths too.
Got any ideas where I should start and/or what I should look for? |
Gardener
Tinkerer
Username: Gardener
Post Number: 15
Registered: 02-2013
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, April 30, 2013 - 09:06 am: |
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I am eager to hear expert opinions on this matter. For the longest time I paid no attention to 80-200 type zooms, but now that I no longer see them just as a cheap source of fasteners I realized that the infinity focus shifts as I zoom in and out. At first I thought I screwed up putting the lens together, even though it is a very simple design, but now I that I've seen the same behaviour on lenses that appear unmolested I suspect it to be a design feature. I mean these zooms are varifocal, so focus must shift at every distance, including the infinity, right?
My personal experience is opposite to yours though, that it is if I set correct infinity at the long end and zoom out infinity focus is reached earlier and at the wide end the lens goes well past infinity. It could be that your Tamron is a different design with focus shift in different direction, so I guess you could set correct infinity focus at 80 and have it at random point at 200 but would that be more convenient? |
Cooltouch
Tinkerer
Username: Cooltouch
Post Number: 260
Registered: 01-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, May 01, 2013 - 04:48 am: |
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Perhaps I wasn't clear enough because I don't think you understand what's going on. The Tamron 80-200/2.8 LD is a push-pull zoom and is not a varifocal lens. Focus should not shift as it's zoomed in and out. And infinity should remain at infinity regardless of the lens's zoom setting.
But what's happening with this lens is infinity has gotten way out of alignment, such that the farthest out it will focus at 80 millimeters is about five or six feet. Even if I push out just a small amount from 200mm, infinity shifts noticeably and I cant get anything located at infinity in focus.
I just examined the lens thoroughly under a strong light, and I can detect nothing amiss internally. Strange. |
Gardener
Tinkerer
Username: Gardener
Post Number: 16
Registered: 02-2013
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, May 02, 2013 - 09:13 am: |
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I think I understood you perfectly well, but are you 100% certain the lens is parfocal? Just because it is a push-pull does not mean focus won't shift as you zoom in and and out. In any case - obviously your infinity is off - probably someone reset it so it would reach infinity at 200 when focus hits limiter. How to set it back? Does the front element rotate when you focus? If yes, it is probably sitting on a helicoid and it's range is limited by the front group housing. Sometimes you need to unscrew the group from the helicoid and find proper thread alingment, sometimes you can just loosen the housing set focus and tighten it back, so I think your way in through the set screws. Also - peel of focus grip and check what's under it.
But what I believe will happen is you will gain infinity at 80, but at 200 it will focus well past it. |
Gardener
Tinkerer
Username: Gardener
Post Number: 19
Registered: 02-2013
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, May 02, 2013 - 12:02 pm: |
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PS. Of course I am crazy and simply need to learn how zooms work. |
Cooltouch
Tinkerer
Username: Cooltouch
Post Number: 261
Registered: 01-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, May 03, 2013 - 11:04 am: |
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I am absolutely 100% positive that the Tamron 80-200 f/2.8 is parfocal. Like almost all other 80-200s out there. A varifocal lens is usually easy to tell apart from parfocals just by looking at it because of the lines scribed on the barrel, indicating hyperfocal settings. They are way different from the lines you see on a parfocal lens -- all the lines are grouped together on one side of the infinity line or the other, but not both . . . because it's varifocal.
Probably the biggest problem with your suggestion -- assuming it works at all -- is the focusing collar stops at infinity. Unlike some long primes, it does not continue past infinity. So if I needed more lens throw, the limits of the collar movement prevent it.
Yes, like almost all zooms, the front element rotates. It is the extension/retraction of the barrel in which the front group is mounted that provides focus.
I have repaired lenses before, including zooms, where, when it came time to put them back together, I was maybe one or two threads off on the helical. Usually I found the correct one through trial and error -- often finding that the marks I scribed to indicate correct reassembly were useless, or of limited value.
Anyway, because this lens is supposed to focus at infinity at the same barrel rotation, regardless of zoom extension, I don't think it's the front group that is the culprit. All it's doing is moving in and out, just like it's supposed to.
You know, it just came to me. You know what this lens is behaving like? Like it's stuck in Macro mode. But the Tamron SP 80-200/2.8 doesn't have a macro mode. So, so much for that idea.
I suspect there's an inner group, which assists with focus, that's off its track somehow, causing this problem.
Here's a link to this lens at the Adaptall-2 website:
http://adaptall-2.org/lenses/30A.html
In addition to the three-element focus group, it has a five-element variator group and a three-element compensator group. I suspect it's one of those groups that's the culprit. |
Cooltouch
Tinkerer
Username: Cooltouch
Post Number: 262
Registered: 01-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, May 03, 2013 - 01:03 pm: |
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Regarding my comments on repairing lenses, I left out some stuff. When a lens is reassembled with the helical threads off, the focus is off across the board by the same amount. That's not what's going on here. This lens can focus at infinity *only* at 200mm. |
Br1078lum
Tinkerer
Username: Br1078lum
Post Number: 545
Registered: 11-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, May 03, 2013 - 07:47 pm: |
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Thanks for the Tamron link. I've never come across that before.
PF |
Gardener
Tinkerer
Username: Gardener
Post Number: 26
Registered: 02-2013
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, July 21, 2013 - 01:09 am: |
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A sudden late thought - are you using it on an adapter? Are you sure focal plane distance is correct? |
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