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06-13-2009, 12:25 AM   #1
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Pentax fa 80-320 4.5-5.6 - can't infinity focus?

Hi All, I'm hoping one of you knowledgeable types can help me find a clue...

I just received the above-mentioned lens, and was futzing around tonight, taking shots of the moon. Interestingly, I cannot focus (auto or manual) this lens on the 2/3 full moon. My DA 50-200 lens does just fine, and my old Tamron 70-300 also had no problems getting a sharp moon shot, on my old K100D.

I'm currently using a K2000 body.

Anyone with any thoughts on this? Or is it just one of those things, and I'm SOL?

Thanks for any input!

06-13-2009, 05:06 AM   #2
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You do have the A setting on the lens locked in ??????
06-13-2009, 09:37 AM   #3
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QuoteOriginally posted by wwwmorrell Quote
You do have the A setting on the lens locked in ??????
Thank you for the response. Yes, the A setting was locked in.

I failed to mention that I am able to MF on the moon at about 80mm, although AF won't work.

A few quick tests this morning, I can AF on a tree about 200 ft away, but not on a variety of subjects beyond that.

All of this is at 320mm.
06-13-2009, 11:36 AM   #4
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It can be adjusted, but I can't find a good enough photo of the front of the lens to say exactly how. From the optical diagram, it looks like the front three elements form one lens group, and I think that group moves in and out to adjust infinity focus. I've done this on a couple of later zooms and that group was held in by threaded rings with slots in them for a spanner wrench. The outermost slots, if there are concentric rings. The rings thread into the filter threads. Loosen the ring, mount the lens on a camera, and try to focus on a distant object at 320mm. Then rotate the group a little and try again. You should check at 80mm before tightening everything; the middle range will be fine as long as both ends work.. AF logic is supposed to be happier if the lens can go past infinity slightly, so I would adjust for that. The group/ring may already be loose, causing your issue. Unless the ring is gorilla-tight, this should be a simple fix.

06-13-2009, 04:31 PM   #5
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QuoteOriginally posted by Just1MoreDave Quote
It can be adjusted, but I can't find a good enough photo of the front of the lens to say exactly how. From the optical diagram, it looks like the front three elements form one lens group, and I think that group moves in and out to adjust infinity focus. I've done this on a couple of later zooms and that group was held in by threaded rings with slots in them for a spanner wrench. The outermost slots, if there are concentric rings. The rings thread into the filter threads. Loosen the ring, mount the lens on a camera, and try to focus on a distant object at 320mm. Then rotate the group a little and try again. You should check at 80mm before tightening everything; the middle range will be fine as long as both ends work.. AF logic is supposed to be happier if the lens can go past infinity slightly, so I would adjust for that. The group/ring may already be loose, causing your issue. Unless the ring is gorilla-tight, this should be a simple fix.
Thank you for that wonderful info! Since my lens is still within the return period, I'm going to send it back, though.
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