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04-30-2010, 01:05 PM   #1
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Pentax 85mm f/2 -- does yours focus past infinity?

I recently bought an M85/2 here on the forums, and while it's in pristine condition, it focuses past infinity at the infinity range marker. What seems odd to me about this is that the MFD does not seem to be affected. Has anyone experienced a similar issue with their 85/2?

04-30-2010, 01:10 PM   #2
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QuoteOriginally posted by deadwolfbones Quote
it focuses past infinity at the infinity range marker.
It is normal for any lens to have the focus ring a bit past infinity. It is part of the tolerance.
04-30-2010, 02:10 PM   #3
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I seem to recall that same deal with my 85 f2. I no longer have it, so I can't absolutely confirm, but I think there WAS a little wiggle past infinity, no issues with the lens.

Enjoy it!

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04-30-2010, 02:18 PM   #4
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QuoteOriginally posted by SOldBear Quote
It is normal for any lens to have the focus ring a bit past infinity. It is part of the tolerance.
When the focus ring is turned all the way toward infinity, the marker is lined up perfectly with the infinity symbol.

At this point, when looking through the viewfinder, the lens is focused past infinity by a decent chunk. To get true infinity focus, I have to turn the focusing ring back toward the MFD by a bit.

This seems like some kind of alignment flaw to me. None of my other lenses behave this way.

But as I said, the fact that the MFD seems correct is baffling. But maybe I'm just measuring the MFD incorrectly?

04-30-2010, 02:24 PM   #5
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I'll check my 85 this weekend. FWIW, my Voigtlander 180 f4 Lanthar used to do the exact same thing...
04-30-2010, 02:25 PM   #6
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what is the lens focused on if it's past infinity?
I don't mean to come across as sarcastic or what... just don't get it... never thought there is something past infinity
04-30-2010, 02:39 PM   #7
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QuoteOriginally posted by axl Quote
what is the lens focused on if it's past infinity?
I don't mean to come across as sarcastic or what... just don't get it... never thought there is something past infinity
Yeah, the concept is kind of to me as well, but basically it means that you have to focus it back toward the MFD to get infinity focus. If you point the lens at a cloud, say (or a mountain), and focus it all the way to infinity it's just blurry. Gotta pull it back a bit to get the cloud sharp.

I've heard of focusing rings turning past the infinity marker, but I've never heard of one that stops precisely at the infinity marker, but is focused beyond infinity at that point.
04-30-2010, 02:45 PM   #8
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QuoteOriginally posted by axl Quote
what is the lens focused on if it's past infinity?
Nothing. That's the problem.

I have to say that having a focus ring that goes too far is better than one that doesn't go far enough.

04-30-2010, 02:56 PM   #9
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QuoteOriginally posted by deadwolfbones Quote
When the focus ring is turned all the way toward infinity, the marker is lined up perfectly with the infinity symbol.
....
This seems like some kind of alignment flaw to me. None of my other lenses behave this way.
Now I know what you mean.

Yup. It is an alignment problem. The lens has a "stopper" screw; over time, the ring just gradually slips pass the stopper. If you are handy and brave, you can open up the lens and adjust the marker. I've done so to several lenses.
04-30-2010, 10:16 PM   #10
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Many lenses will focus past infinity a slight amount by design. I have heard that this is to accomodate infrared film. The infinity focus can be easily adjusted. Having a split prism focusing screen makes it a cinch. You just loosen the screws holding the barrel with the distance markings to the focusing helix and turn it to line up with infinity when the lens is focused at infinity. Then just retighten the screws.
04-30-2010, 10:32 PM   #11
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QuoteOriginally posted by deadwolfbones Quote
When the focus ring is turned all the way toward infinity, the marker is lined up perfectly with the infinity symbol.

At this point, when looking through the viewfinder, the lens is focused past infinity by a decent chunk. To get true infinity focus, I have to turn the focusing ring back toward the MFD by a bit.

This seems like some kind of alignment flaw to me. None of my other lenses behave this way.

But as I said, the fact that the MFD seems correct is baffling. But maybe I'm just measuring the MFD incorrectly?
The typical Pentax prime can be adjusted so that the infinity stop is right at infinity. If someone reassembled the lens so the focus helix was screwed in as far as it will go, then attached the focus ring with the orange diamond pointed at infinity, the lens would work like yours. If the focus ring isn't loose, it's most likely just adjustmment.

I have experimented with one lens (M50/1.7) to see if the MFD could be extended or shortened by a meaningful amount. I couldn't consistently measure any worthwhile difference - it might have been 12mm/0.5 inch at the most.

It's not a big deal to fix it. I can explain how with photos if you want. It's also not a big deal to leave it as is.
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