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Robert Bonn
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, January 30, 2006 - 02:26 pm: | 
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The lens: Pentax-A, 50mm, f/2. Very clean, and aperture working fine.
The problem: The focus ring feels stiff when it turns, and the focus itself does not change. In other words, you can turn the ring, with some difficulty, but the lens doesn't focus.
I got this lens for very little money, and have no sentimental attachment to it, so I'm thinking I might try fixing it myself. My goal is to make it a nice lens for a user, and not turn it into a bucket of parts! So I'd appreciate any tips.
By the way, this is my first visit to this site. I'm very impressed with the knowledge and helpfulness here, and I've already learned a lot. Thanks in advance. |
rick oleson
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, January 31, 2006 - 10:04 am: |
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If I recall correctly, Pentax attaches the focus ring to the actual focus mechanism by 3 screws that are reached from the front, behind the lens ID ring. The screw heads clamp down on the ring so you can adjust the infinity position - if they get loose it slips.
This still leaves you with the problem of why the focus is so stiff. old lubricant should not make it THIS stiff. I have seen at least one case in which an impact to the front end of a lens caused the focusing helical to jump over a thread on one side, resulting in a cross-thread that jammed it solid. This amount of cross threading would be visible in the form of the front end of the lens barrel not being parallel to the mounting flange. |
Glenn Middleton
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, January 31, 2006 - 06:07 pm: |
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I'm reading the problem somewhat differently, but I might have misread the situation. Robert seems to be stating that the focus does not change with rotation of the ring. So is the helical jammed and the stiff focus ring is due to the ring turning against the three tightened retaining screws? He does not state if the focus ring has the correct angle of rotation. If the screws fit in slots the rotation should be limited, I would have thought. Perhaps Pentax took the cheap fix and the screw heads only bear on the inner edge of the ring, allowing slippage of 360 deg. |
Robert Bonn
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 01, 2006 - 07:36 am: |
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Thank you for these two responses. I took the lens partially apart last night, and you're both correct.
As Rick said, the focus mechanism has 3 screws, located behind the lens ID ring. The screws were slightly loose, but just slightly. Whether this was enough to cause the problem seems doubtful. I could try tightening them down a little more, but it sure doesn't look like that will solve it.
As Glenn suggested, the heads of the three screws appear to ride in a small groove on the inside of the focus ring. The screws don't actually clamp down on anything, so although the focus ring turns, it's basically just scraping along the screws. That would certainly explain why the lens won't focus.
Glenn, I'm not sure what you're referring to when you mention the angle of rotation. In case this helps, the focus ring is in the correct position on the lens, compared to other Pentax lenses I've seen. It turns in the correct directions, and stops at the right places. (Infinity lines up with the little diamond shape in one direction, and the closest focus lines up when it's turned the other way.) A tab on the focus ring prevents the ring from turning 360 degrees. The ring is not in a crooked position or anything like that.
Both of you suggest that the helical is jammed, and although I'm a novice, that would be my thought as well. The front end of the lens appears to be parallel with the lens mount. (I didn't specifically check that, but I can say that the front of the lens is not noticeably out of parallel.)
If the next step is to unjam the helical, how do I do that?
And by the way -- as you can tell, this is my first attempt at lens repair, and I got it back together again without screaming. While reading this forum's archives, I saw that quite a few people ruined the first things they tried to fix, so I'm feeling pretty good today. I look forward to actually fixing something, though, rather than just looking around. |
Glenn Middleton
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 01, 2006 - 09:23 am: |
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Robert,
Not having an example of this lens, I was just trying to find out if the focus ring had its proper limits of rotation.
These three screws, how are they spaced from each other? 120 degs, or are they in a single slot relatively close to each other? To lock the ring on the lens, I would expect them to be at 120 degs and the screws to have 'pan heads' that tightened onto the ring. Your description has me seeing the heads fitting in the slot, thus acting as some kind limit stops.
If the screws are clamping screws, one reason that the ring is still slipping could be that the clamping surface has worn and the screws are bottoming in their respective holes.
Another thing, you say lens does not focus - this being checked on a camera I suppose - but are you sure that the actual rear group is not moving? If lens has been assembled wrongly at some point, you could get the 'non focus' problem.
At moment I cannot get round the fact that focus ring moves over its correct limit. Well not exactly true, I can give an explanation for my Canon FD lenses.
If rear group does not move in and out, does it rotate when you turn the focus ring? I may be completely off track but if anti rotation screw/peg or whatever is missing from the helix, it is possible that you will not get in/out movement but only rotational movement of focusing group - sometimes there can be a very small amount of in/out, but this can be hardly noticeable.
If the helix has seized it is possible that the anti rotation device has been sheared off by using brute force.
Sorry if this seems overly complicated and unhelpful, always easier with a similar beast or diagram infront of you.
Glenn. |
Robert Bonn
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 01, 2006 - 07:57 pm: |
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Glenn, I'll try to answer your questions in the order you asked them.
1. The three screws are at 120 degrees. They're very small, of course, but they do have slightly "pan" heads, and in addition to that, they have small washers under them. My sense is that they exert sideways pressure on the focus ring. The groove inside the focus ring is quite small, and doesn't seem deep enough for screws to clamp down onto.
2. Yes, I checked the focus on a camera. When I turn the focus ring, no change is visible in the viewfinder.
3. The rear lens group does not move in or out when you turn the ring, and it does not rotate either. Same with the front lens group -- no in-out movement, no rotation.
Does this help analyze the problem? If I can provide any additional information, please let me know. Thank you for taking your time and lending your expertise on this. |
Glenn Middleton
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 01, 2006 - 08:39 pm: |
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Robert,
Ok, suppose I should do a diagram but I think this will put me in the picture - if a capital L represents the focus ring and a full-stop the screw, I picture you have this L. ie all the screws bear on the inside edge of the focus ring and the washers fit onto a machined ledge or groove around the inside edge.
Tightening the three screws down should do one of two things; the focus ring will be jammed solid or it will rotate and move the helix. If it does rotate I think it will be very stiff.
I fear that the only solution will be to strip the lens down completely and check and clean everything, before re-greasing.
The problem is as Rick says, old lube on these Pentax lenses does not normally cause this degree of seize up. Perhaps it has been reassembled wrongly, however I have no depth of knowledge on this particular lens. Thus I am as you, working in the dark.
Glenn |
Robert Bonn
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, February 03, 2006 - 11:14 am: |
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Thanks for the information, Glenn.
I took another look inside the lens, and tightened down the three screws. The focus ring is now jammed solid. (I should add that while I had the lens partially apart, I wasn't able to rotate the helical by hand either. I would think that would ordinarily be possible, but I've never been inside a lens before.)
So I think this points to the need for a more complete disassembly.
Does this seem like something that I could take on myself? I wouldn't need step-by-step instructions, but I would certainly need some general guidance. (For example: would it be necessary to remove all the stuff that's located under the lens mount? I looked in there, and it was plenty scary to a novice.)
I have a feeling that this repair is beyond the amateur level. If so, perhaps the best thing for me to do is give up. I'm considering listing the lens on eBay as "for parts or repair." It may be worth fixing, and it certainly has mostly perfect parts.
If, on the other hand, it isn't as overwhelming a task as I think, I'm open to ideas.
Thanks again! |
Glenn Middleton
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, February 03, 2006 - 02:48 pm: |
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Robert,
Sorry I know nothing about Pentax lenses with this mount. Having worked on a number of other makes, I am sure that I could eventually get it apart. Still that does not help you solve the problem. You will have to strip the lens down so that you can free the helical, but if it is something simple like a loose or displaced screw, you might find this before you reach the fully stripped down stage. On the other hand if it is the old grease that is 'bonding' it all together, you will have to get the helical out so that you can soak it in suitable solvent.
With a parts diagram I am sure you could sort this out, but I have been unable to find any official repair documentation that would allow me to help if you ran into problems.
I do have a niggling thought that this problem may be due to DIY gone wrong. As Rick mentioned, old lube does not normally seize up solid - well not on Pentax lenses of that era, old Eastern Bloc stuff is another matter - so you should be able to get a little movement. If it is something broken or damaged, you will need spares for reassembly.
If lens was really little money you might consider having a go and use it as a practise piece. It will set you up for other Pentax lenses and in all probability you will get it sorted anyway.
There are people here that really know Pentaxes, they will be able to guide you better than I can.
Glenn. |
Bob Potter
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, March 05, 2006 - 09:16 pm: |
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Robert, I read about your situation. I also have a 50mm lens in which I received from my sister-in-law. She fell and broke her arm trying to protect her Pentax k-1000 camera when she dropped it. After she recovered she tried to use the camera, but it would not work. This past summer we were visiting her and I was photographing the family when she asked me if I would like to have hers. I accepted her offer. As she said it wouldn't work. It also had a different lens with the camera. So I tried that lens on the camera and it worked just fine. I offered the camera back, but she declined.
I have studied that lens for some time and this week-end decided that I would be out nothing if I couldn't fix it. I decided to do some research and happened onto this page. I took the lens apart to the point that you had and studied it for some time. That threaded aluminum ring inside of the focasing ring has to turn with the lens adjusting ring. That is what moves the lens in and out. Turning the focasing ring to the right can seat that aluminum ring if it slips and then catches again when turned to the right again. (I beleive that is what has happened to yours). I determined that and realized that the aluminum ring has to turn clock wise in order to thread the lens outward. If you remove those three screws, you can remove the lens focasing ring. There is a small notch in the top side of this ring. You can take a small screw driver and insert it into this notch and rotate the ring to the left. I had to force upward pressure on the lower edge of the ring to free it, but it came loose.
Adjustment is a little tricky. The ring needs to be turned down nearly all the way down, but not so far that it will bind when you tighten the three screws down again on the focas ring. Also, I had to re-adjust the position several times because I kept getting confused as to what position the focas ring had to be in before I tightened it down. There are two copper right angled peices screwed to the back side of the lens under the diaphram adjustment peice. If you remove them, you can screw the lens element out to fully expose the aluminum ring. (Be careful as there are tiny parts that can possibly fall out, especially if you remove the apature ring, which I feel that you do not need to do). I have one tiny part left over, but sinse my camera is fully manual and the lense is an A lens, I don't beleive that I need it. It dropped out after I pulled the apature ring off.
I had to reassemble the lens three times because I failed to get everything positioned properly. If the aluminum ring is not threaded down far enough, you will not get the lens to focus to infinity. Also,you can get positioned in a way so that you can not focas down close. So it will take some patience on your part to put it back together. Those copper peices prevent the lens from rotating and force it to move in or out. Good luck, Bob |
Chetan_dighe
Tinkerer
Username: Chetan_dighe
Post Number: 1
Registered: 02-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, February 26, 2007 - 10:17 am: |
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Hello Robert,
Just curios - have you solved it by now?
I have two pentax lenses - 50/1.4, 24/2.8. Both are fairly similarly built. I take them apart from both sides from time to time, to clean up the fungus. Haven't really got into any trouble. Just remember the order in which you unscrew them ...
Cheers,
Chetan |
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