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07-01-2011, 03:21 PM   #1
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A 50mm /1.2 won't focus to infinity

Hey guys I just got an FA31 that wouldn't focus more than 20 feet away, I am in the process of returning it but it got me worried so I checked the rest of my collection. Everything focuses spot on to infinity or a little past (my A* 300mm focuses waaaay past!) but my new 50 f1.2 can't quite make it to infinity.

Here it is stopped down a little:




Click image for original size


Is there an easy way to allow the lens to focus a little bit farther? I'm not averse to taking things apart but I've heard stories about how many little bits hold this lens together

07-01-2011, 03:50 PM   #2
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Sorry to ask a silly question, but are you sure you're not passing infinity focus on the lens? Most lenses allow for some focus ring freedom past infinity focus. You mention focusing past infinity, which doesn't make sense but I'm assuming you mean passing the infinity focus point (since there's no subject we can photograph that is more that infinity distance away!)
07-01-2011, 03:54 PM   #3
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Yes, I backed it off a bit and the focal plane continues towards me. As you can see the boat in the foreground has focus when the lens is focused to the infinity mark.

And yes, your assumption is correct about my meaning of "focusing past infinity".

Last edited by RXrenesis8; 07-01-2011 at 05:53 PM.
07-01-2011, 06:40 PM   #4
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Wow, that's some intense fringing going on...

And having said that, notice how there's purple nearer and green further? You can tell where the focus plane is, roughly, though I'm not sure of all the technical terms to describe this. Looking at the pine trees sort of next to the roof on the left of the picture, you can see the purple fringe in front of the focus plane, the branches in focus, then the green fringe past the focus.

You may have to disassemble it, from the front, which is fairly straightforward and should be like the other A lenses. Just don't mess the with back of the lens...what a pain :ugh:

What's strange to me is that you have not one, but two lenses that won't hit infinity. Were they both new in box or something? Did the previous owners ever use them?

07-01-2011, 06:49 PM   #5
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QuoteOriginally posted by ryan s Quote
You may have to disassemble it, from the front, which is fairly straightforward and should be like the other A lenses. Just don't mess the with back of the lens...what a pain :ugh:
I think the 50/1.2 lenses are different. I don't have one and I'm just going on a vague memory of a discussion I can't find, though.
07-01-2011, 07:13 PM   #6
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Hopefully it has that little ring inside the filter threads (where the name went on M lenses)...if not, then there may be no choice to take the back off...

In that case, I wish the OP the best. I passed on a lowly, cheap A50/1.7 with a stuck aperture ring/A position just because I didn't want to put it back together again.
07-01-2011, 07:14 PM - 1 Like   #7
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Dissasembly to adjust the focus on the pentax 50mm f/1.2 K series lens is pretty easy,this is the method I use:

first thing first: remove the ring around the filter thread and set the lens focus to 0.45m (you can remove the filter thread, it will give you more space to work, but it isn't necessary) - which will give you access to the four screws that connect the focusing ring to the helicoid. Set the lens focus to 10m, loosen all five screws (for god's sake don't take them out entirely) and set the focusing ring to 5m and tighten one screw. Now for the next step I prefer to use Live view to identify where I want the infinity stop to be located, but if you don't have live view you're going to have to spend a lot of time taking photos and running back to your pc to ascertain if the lens is focusing where you want it to. Because the LCD's on cameras aren't that good at telling you where the point of critical focus is - even with fast lenses.
07-01-2011, 07:18 PM   #8
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QuoteOriginally posted by ryan s Quote
Were they both new in box or something? Did the previous owners ever use them?
Naah, the 31 came from Adorama and the 50 I received yesterday second hand. It MIGHT still be under warranty, it's from a newer batch but I wouldn't hold my breath.

If it's not a simple and well documented procedure I'll probably just send it in to get serviced.

07-01-2011, 07:32 PM   #9
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QuoteOriginally posted by Digitalis Quote
first thing first: remove the ring around the filter thread and set the lens focus to 0.45m (you can remove the filter thread, it will give you more space to work, but it isn't necessary) - which will give you access to the four screws that connect the focusing ring to the helicoid. Set the lens focus to 10m, loosen all five screws (for god's sake don't take them out entirely) and set the focusing ring to 5m and tighten one screw. Now for the next step I prefer to use Live view to identify where I want the infinity stop to be located, but if you don't have live view you're going to have to spend a lot of time taking photos and running back to your pc to ascertain if the lens is focusing where you want it to. Because the LCD's on cameras aren't that good at telling you where the point of critical focus is - even with fast lenses.
Are you going to give me the other things I need to do now, or wait until I have completed the first step?
07-01-2011, 07:44 PM   #10
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QuoteOriginally posted by RXrenesis8 Quote
Are you going to give me the other things I need to do now, or wait until I have completed the first step?
well it might be useful for you to know this..
07-01-2011, 07:48 PM   #11
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QuoteOriginally posted by Digitalis Quote
well it might be useful for you to know this..
Haha, please don't tease me! Continue and finish your explanation of the procedure!
07-01-2011, 07:54 PM   #12
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well assuming you have actually have your lens focusing on the point you need it to. To make the focus adjustment permanent simply re-tighten the other three screws onto the focusing helicoid - replace the front lens bezel - and the filter thread mount, if you removed it.

Last edited by Digitalis; 07-01-2011 at 07:59 PM.
07-01-2011, 07:58 PM   #13
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OK, I cannot budge that plastic ring inside the front filter threads, any advice?
07-01-2011, 08:03 PM   #14
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QuoteOriginally posted by RXrenesis8 Quote
OK, I cannot budge that plastic ring inside the front filter threads, any advice?
you can't move that ring, I'm talking about the ring with the writing " SMC pentax-A 1:1.2 50mm" on it. That ring turns counter-clockwise and will give you access to the screws that will allow you to adjust the focus.
07-01-2011, 08:17 PM   #15
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What tool do You use when you take that off? Because mine is on there pretty good.
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