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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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09-18-2011, 08:06 AM   #2
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Very good, thank you for the DIY. Coming from the DIY automotive world - we could really use more of these.

I usually set the infinity point for the moon and the stars (very easy target with the live view) on longer lenses as well.

One tip from me - I like to wear latex gloves as the internal elements are very hard to clean properly if skin oils get on them.
09-22-2011, 07:17 AM   #3
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You're right, Michael. It's highly recommended to use latex gloves. I would also like to add that I also have a film camera now, and I re-checked the focus adjustment on that. With the microprism focusing screen, you can easily set the correct focus. I saw that I've overcompensated a little bit, maybe I'll open the lens again some day and tune it to perfection.
01-21-2012, 02:54 AM   #4
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This looks like a great procedure for bringing all my long telephotos to the same infinity focus point. I've understood that part of the reason why these long lenses needed the focus setting away from the stop was due to the change in the lens dimensions as it was used in different temperatures. This makes sense for very long lenses, but I'm not too sure about a 200mm.

01-22-2012, 06:38 AM   #5
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QuoteOriginally posted by smigol Quote
This looks like a great procedure for bringing all my long telephotos to the same infinity focus point. I've understood that part of the reason why these long lenses needed the focus setting away from the stop was due to the change in the lens dimensions as it was used in different temperatures. This makes sense for very long lenses, but I'm not too sure about a 200mm.
Indeed, thermal expansion isn't really noticeable on a 200mm lens (unless maybe if used in extreme environments). I think that for this lens, the wrong infinity focus was simply due to improper previous disassembly and reassembly. Whoever took the lens apart before me probably forgot to mark the 'true' infinity focus position.

If you want to adjust your lenses, search the web for additional info, because not all Pentax lenses are disassembled exactly the same way.
09-03-2012, 06:29 AM   #6
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I have found this question about infinity error in lenses on literally dozens of forums. If you find that your lens can focus slightly out of focus when you wind the lens all the way to the infinity stop - leave it alone there is nothing wrong with it. Modern lenses are made out of a number of different materials: brass, bronze, different alloys of Aluminium and obviously glass of varying thicknesses. All of these materials expand and contract at different rates when the temperature changes, so infinity can be at a microscopically different place at different times of the day or night. Also most zoom lenses are what is known as VARIFOCAL lenses so infinity changes every time you zoom the lens. the ability to focus "past"infinity is the way modern manufacturers counter this. The best advise I can give tibi is put it back the way it was, if you can.

Last edited by baldrick; 09-04-2012 at 02:59 AM. Reason: short fat fingers
09-04-2012, 08:14 AM   #7
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Thanks for your comment, Baldrick. I would like to clear some things up. "Modern lenses" yes, often can focus past the infinity mark, and it is absolutely no problem. You have autofocus anyway, and the autofocus doesn't care about the distance markings on your lens, it just finds the correct focus. The same thing happens if you focus manually, but with a split-screen or a microprism (like on old film bodies, or if you've changed the focusing screen in your camera): you just turn the focusing ring until your image is no longer split.

HOWEVER, my lens was unable to turn until the real infinity, it was off by a lot! And you simply couldn't turn the focusing ring any further, because it has a hard stop at infinity. This is why all the pictures of distant objects looked blurry, because the lens could only be focused about as far as 100-150m, and on long lenses, 150m can't be approximated with infinity, the depth-of-field is too small.

So my advice is:
If your zoom lens focuses past the infinity mark, don't bother fixing it, because zooms are often varifocal (focus distance depends on zoom), as baldrick said.
If your fixed-focal lens focuses past the infinity mark, again, no big problem, because you CAN reach infinity somewhere in the focus ring travel. The only time when it's useful for the focus travel to stop exactly at infinity is low-light photography and especially astrophotography, where there isn't enough light to judge focus, and you just need to focus at infinity. Generally, it's actually better to allow the lens to focus a little bit past the infinity mark, to allow for thermal variations (especially true for very long lenses, 200mm+).

BUT if your fixed-focal or zoom lens CAN'T reach true infinity, meaning it can only focus closer than infinity before the focus travel reaches its hard stop, then you have a problem. This problem usually occurs after someone has taken apart the lens for servicing, but he/she hasn't reassembled it exactly the same way. Get your lens reassembled properly.
A similar problem arises when using M42-K adapters that are too thick: the lens can't be brought close enough to the sensor to allow focusing at infinity. Look for an adapter that's thinner.

If you have a new lens that can't reach true infinity, take it back for a warranty exchange/repair. The problem is easy to spot, you can't get in-focus shots of distant objects no matter how you focus, but the shots of stuff that's closer look nice and sharp.
09-12-2012, 04:22 AM   #8
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My apologies tibi, I missed that point in your original post.
Simmo.

10-01-2012, 07:30 PM   #9
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Polarizer caused infinity focus issue with my k2.5/200mm lens

So... I was having trouble with my 200mm infinity focus and came across your article. Thank you!

But a strange thing happened. I made the adjustment, tested and reassembled the lens. The last stage was putting the hood on, which had my Vivitar polarizer mounted on. Looked through the lens - no abilty to focus at infinity!! Thinking I was careless with my reassembly, I disassembled again. Tested again. Reassembled again. Again, no infinity focus.

Finally it dawned on me. Something was wrong with that polarizer. When I tested, I did not have the polarizer on. When I finished the reassembly, I did have the polarizer on. Once I removed the polarizer, I had my infinity focus!

I then tried the polarizer on my M*300 lens, but because that lens can focus PAST infinity, the polarizer reduced the focus to infinity - not past infinity.

Needless to say, the polarizer is now relegated to my box of unused filters. Does anyone know why that polarizer was causing the reduced distance?

I use k1000 film cameras with these lens. The polarizer is a Vivitar circular - rather than linear. It's a newer polarizer as compared to my K 2.5/200mm old lens. Feedback welcome!
10-03-2012, 12:52 AM   #10
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Great to see that this article is useful!

Most polarizers, except really-really good ones, are quite far from being perfectly flat. That's because they're harder to make than other filters, they have more layers and they are made of stuff that's more difficult to work with. Small errors in the flatness of each layer accumulate. When used on lenses with short focal lengths (say, 50mm or less), you don't really notice the effect. On longer lenses, the magnification of the lens amplifies any errors in the filter.

I, too, noticed this problem. I had a cheap, no-particular-brand (ok, Vivitar IS a brand) circular polarizer. On the 18-55mm, I never noticed any problems. On the 50-200mm, no matter how I turned the focus ring, I couldn't get a sharp picture. Took the filter off, everything was back to normal again.

I once stumbled upon a test of various polarizing filters. Among other things, they test the homogeneity of the filter. Here's the link:
Polarizing filters test - Introduction - Lenstip.com
Even some filters from very respectable brands had problems with homogeneity.

So, an addendum to the infinity focus fixing article: take the filters off when doing this!
08-16-2014, 12:06 PM   #11
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Well, there is an easier way to adjust focus on this lens, just shift up the rubberized grip, and three very tiny screws show up.
When loosened (removing not required) you can focus without rotating the distance scale, and so you can adjust infinity focus.

Do not cut the rubber grip, just break the glue by putting a thin screwdriver below the rubber etc.
The rubber can be glued back, or what I do, use double sided sellotape.

It is very uncommon that focus or zoom adjustments must be done with the optics part of a lens disassembled, I never encountered such a lens.
08-17-2014, 08:04 PM   #12
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Actually, the procedure described is how focus is adjusted for that lens and hundreds of others made from the 50's through the 90's, especially fixed focal length lenses.
08-17-2014, 08:30 PM   #13
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Excellent. This should be also referenced/linked in the Lens Repair Database thread.

I would use a JIS driver instead of a Philips Head screwdriver to lessen the change of stripping the screw heads.
11-03-2014, 08:54 AM   #14
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I reset an AF lens to infinity doing the following: (Please read disclaimer below- this may not work for you, I tried it as a last resort and it did work for me)

It is worth hanging on to an outdated body of your brand just to use for lens adjustments, so you don't have to worry about damaging your camera)
Set your (outdated dslr) camera for a LONG EXPOSURE
Focus on Infinity (even though lens won't focus to infinity)
Release the shutter
Removed lens before exposure completed.
Shut off camera
Replace lens.

The camera body and lens I tried it with was an Olympus. I found this 'fix' method 'googling' the problem.

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.
11-05-2014, 06:48 AM   #15
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QuoteOriginally posted by Pentaxaholic Quote
I reset an AF lens to infinity doing the following: (Please read disclaimer below- this may not work for you, I tried it as a last resort and it did work for me)

It is worth hanging on to an outdated body of your brand just to use for lens adjustments, so you don't have to worry about damaging your camera)
Set your (outdated dslr) camera for a LONG EXPOSURE
Focus on Infinity (even though lens won't focus to infinity)
Release the shutter
Removed lens before exposure completed.
Shut off camera
Replace lens.

The camera body and lens I tried it with was an Olympus. I found this 'fix' method 'googling' the problem.
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).
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