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How to fix infinity focus on K 200mm f2.5
Posted By: tibi, 09-18-2011, 06:46 AM

Hello to everyone!

I've managed to adjust the infinity focus on my SMC PENTAX 1:2.5 200mm, and I thought I'd share the experience with you, maybe someone else might find it useful, too.
Here's a picture of the lens:
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Before I begin, a legal disclaimer: the technique described here worked for me, it doesn't necessarily mean it will work for you, too. I DO NOT ASSUME RESPONSIBILITY IF YOU DO SOMETHING BAD TO YOUR LENS AS A RESULT OF THE TECHNIQUE THAT I HAVE PRESENTED HERE. IF YOU DECIDE TO DISASSEMBLE YOUR LENS, YOU DO THAT AT YOUR OWN RISK. IF YOU ARE NOT PREPARED TO ACCEPT THESE RISKS, THEN DON'T DISASSEMBLE YOUR LENS. INSTEAD, CONSULT A PROFESSIONAL SO THAT HE MAY DO THE REPAIRS.

How I found out that I had infinity focus issues: because all the stuff that was very far away appeared blurry in the pictures, when the focus ring position was set to infinity. But stuff that was closer (at about 200-300m) appeared sharp, even wide-open at f/2.5, when the focus was set on the infinity mark. Images at infinity got somewhat sharper when I stopped down the lens, but this was only because the DOF increased when stopping down. To achieve true focus at infinity, I would have needed to turn the focus barrel just a bit more, two or three degrees, past the infinity mark, which of course was not possible.

This lens focuses by moving all the optical elements together towards the front (to achieve close focus), or towards the rear (for infinity). So I needed to make the optical elements move towards the rear just a little bit more, to hit infinity right-on.

What you'll need to open the lens:
- a good PH 00 screwdriver (with a long enough blade, so that it will be able to reach into narrow spots), avoid the interchangeable tips screwdriver sets, because their body is too thick and you can't reach inside the lens
- an air pump (like the rocket blower), to remove those few dust particles that inevitably settle on the inside lenses
- a clean workplace
- an object that's very far away, at least 2km, on which you can check infinity focus. A far-away mountain peak, or the Moon, make excellent targets. Avoid looking above hot asphalt, or over rooftops in the summer heat, because this creates a lot of turbulence in the air, and makes it more difficult to check sharpness.

Also, it is advisable that you put a clean cloth under the lens while you work, so that those little screws won't go far if they fall on the table.

Now let's begin the disassembly:

Extend the lens hood and you will see three little screws keeping the hood and the filter ring attached.Unscrew them and remove the lens hood. Don't mix up these screws with those from the lens hood, because they're of a different length.
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Now unscrew the entire front group as a whole (grab onto the bottom-most ring, otherwise you'll unscrew the front part of the group). Remove the front group from the lens.
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Now, for those of you who need to also do maintenance on the aperture blades, this is also how you access the front part of the blades. I don't know how to access the rear part of the blades, as this wasn't my objective, and I only took apart strictly what I needed to.
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Now you'll see three more screws, unscrew these also and remove that metal thing that they keep attached. Be careful not to drop these screws into the lens (keeping the lens horizontal when working with these screws might be a good idea).
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Now, if you look inside the lens, you'll see that the focus barrel (the rubberized thing on the outside of the lens, that you turn to focus) attaches to the focus helicoid (I believe that's how it's called?) by means of three screws. When you turn the focus barrel, it then turns the exterior part of the helicoid (please correct me if the terms that I'm using aren't entirely correct), which then moves the central part of the lens (the one containing the optical elements) forward or backwards. The screws are marked with red (you can also see the reflection of these screws on the inside of the focusing barrel).
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What you need to do now is to loosen these screws so that the rubberized focusing barrel will be able to turn freely, so that it doesn't also move the inner part of the helicoid (which would then cause the optical elements to move forward or backwards). DO NOT UNSCREW THESE SCREWS COMPLETELY, JUST LOOSEN THEM! If you've taken these screws out completely, good luck putting them back in!

Now that you've loosened those screws, you have to turn the focusing barrel a bit, in the appropriate direction, so that you compensate for the previous focusing misalignment. My lens only managed to focus to about 200-300m, so BEFORE I loosened these screws, I set the focusing barrel on the infinity mark, then loosened the screws, then turned the focusing barrel to where I thought the 200-300m distance should correspond (see the picture, where, after loosening the screws, I align the focus mark to be just a little bit before infinity). Then I tightened back the three screws.
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If your lens, unlike mine, focuses PAST infinity (you get the clearest pictures of objects at very high distance when you set the focus to be actually before the infinity mark), then do the following: BEFORE disassembling the lens, make a note of the position on the distance scale where you achieve TRUE infinity focus. Then follow all the steps described above to disassemble the lens. Before loosening the three screws described above, set the focus barrel on what you found out to be true infinity focus. Now loosen the three screws (be careful not to rotate the focus ring while you do this), then turn the focus barrel so that it reaches the infinity mark. Now re-tighten the screws.

Note: it's good to tighten these screws well, so that the focusing barrel won't slip later on, after you've assembled back the lens. However, don't tighten them like a brute, either, in case you'll need to disassemble the lens again for maintenance, in the future.

Now you can assemble back the lens. Test to see if infinity focus is good. Note that on a lens of this focal length, you have to check infinity on a target that's very far away (I recommend at least 2km. The Moon makes quite a nice target)! Also, when checking focus, make sure your diopter adjustment for the viewfinder is properly calibrated (or use magnified live-view, or take several photos to find the exact focus adjustment for the greatest sharpness at infinity). If it needs more adjusting, follow the same procedure again.

Congratulations, you've finished adjusting your lens! Now you can enjoy sharp pictures at infinity, sharp enough even wide-open! Now your only limitations in sharpness are camera shake and atmospheric turbulence.

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11-07-2014, 07:54 PM   #16
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Join Date: Aug 2014
Posts: 76
Thanks for input

QuoteOriginally posted by tibi Quote
This sounds like some sort of "software hack", to fool the camera into thinking the infinity position is elsewhere. I'm not sure how much this fix would help in the case of Pentax AF lenses. If you use a screwdriver to turn the focusing screw inside the lens mount, you'll hit the same hard stops at the ends. If infinity can be reached somewhere during the travel, the AF system of the camera can find it on its own, no need to do any strange meddling. And if the lens consistently front/back-focuses, you should compensate from the Pentax DSLR settings menu (I don't remember where, I've never used the feature, but I know it's present).
Thanks for your thoughts on this. All I know is it DID work for the Olympus lens I tried it on. It just was not tack sharp on my camera at infinity. The camera had a setting to reset lens to infinity when you power on. It still didn't work, so I followed a thread and tried it and it worked.
I knew the lens was capable of far sharper images at infinity, because I used it on my earlier Oly dslrs.

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