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04-06-2012, 03:49 PM   #1
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Focus problems with various lenses

I have an smc Pentax-M 28-50 that has been sitting on my shelf unused for few years. I got this in a batch of items, and probably never used it because it would not focus, because today I decided to take some shots with it to see what the images look like with my K20D and "rediscovered" this problem. It's optically excellent, really clean and fungus free, and the mechanicals and cosmetics are great as well. If it focused, I'd probably really enjoy using it, but really, it's super blurry along the entire, short zoom scale and at all distances, except really close up, which is where the "almost" in the thread title comes from. I seem to have this really weird macro lens, because I can focus at about 2 inches in front of the lens, and then it's sharp! The focus mechanism works, nothing is loose, and the zoom also works.

The way I understand it, it's as if the rear element is too far away from the sensor plane, and when I add a No.1 extension tube to the lens, the focus is even worse. Has anyone seen this type of problem before, not necessarily with this lens? I'd love to get it working, just because I haven't seen any of these around much, they're kind of rare.

The condition of the lens is too good for me to imagine that there is an alignment issue (definitely never been banged or dropped), but I guess that is always a possibility. It's not working anyway right now, so if someone has an idea that would require me to disassemble it somewhat, I'd give it a gingerly try. No electrical contacts, so it can't be too hard, right ;-).

Even photos of what it "should" look like may be helpful - the one on the lens review section looks the same as mine at those settings, as does Bojidar's photo, just from an eyeball perspective.


Last edited by demondata; 04-09-2012 at 11:54 AM. Reason: Change title to more generic focus issues thread
04-06-2012, 04:58 PM   #2
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I don't know the mechanical details of that lens, but I've picked up good looking used lenses before that were similarly messed up without external signs. I sent a 90mm (Leica) that had low contrast and sharpness to my favorite technician who reported that the rear optical group was too close to the film plane. After he did a CLA it was perfect - one of my best lenses.
However, a good CLA will probably be more than the cost of a replacement. I see KEH has that lens, in "excellent" condition, for $59 US.
04-06-2012, 05:13 PM   #3
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It is not just the rear element to sensor distance that matters. The front element or group of elements typically moves forward and back to change focus. If the front glass is to far forward it would cause that too. I recently got a lens off ebay where the front and rear sections of the lens are starting to come apart from each other (arrived very poorly packed and probably took a good smack breaking it internally). It functions fine other wise. Of courses this is a plastic AF lens. Its more like too much play than actually coming apart yet but even a little bit to far forward on the front glass and it is starting to loose infinity focus. Gently pressing rearward on the front ring corrects it. Its probably not more than a mm or 2 movement but it makes a difference.

I have no idea how that lens is built but it could be something as simple as the outer ring that turns the front group has slipped a little and or the stop that controls how far the ring turns in either direction is loose and has slipped a little. Most of the older all metal lenses I have worked on (which granted is a limited number) have had a big heavy spline for the front glass travel which I wouldn't think would have slipped (the one I got has a thin plastic threaded ring to control it so I could see it slipping a thread or two if hit). I guess perhaps if there were enough play in the spline maybe it jumped a tooth it it was smacked hard just right? Could be lots of things but this is another possibility. Loose mount making the lens come a little to far away from the camera?
04-06-2012, 05:23 PM   #4
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Something you might want to try (very gently as you don't want to break something else). If there is a barrel holding the front glass that extends out of the front of the lens when zoomed and appears a separate piece from the focusing ring, zoom where it is furthest out, the hold it in one hand and hold the zoom ring in the other. Try twisting them in opposite directions. Again, do it very gently as there are tiny screws holding things together and you don't want to break anything. If there is an adjustment there that is held by set screws and the set screws are loose (causing the problem perhaps you can get it to adjust a little that way). You might also slip the rubber grip ring off carefully as that can expose such adjustments on some lenses.

04-07-2012, 02:08 PM   #5
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Thanks, TomB_tx, for referring me to the KEH example as a good reason not to do a CLA - I thought as much, that's why I was prepared to mess with it a bit myself. As for the suggestion about twisting the two parts in opposite directions, ripit, I did that and everything look/feels fine. When I look into the front of the lens, I see the second group move closer when I zoom in from 28 -50, and the front part definitely moves further away from the rear part when I turn the focus ring. Blurriness is severe, so something is really out of place, but it's "best" at 28mm, and worst at 50mm. Funny thing is, even though the front part extends when I turn the focus ring, almost nothing happens to the degree of blur or lack of focus. Everything stays almost equally blurry. I have a feeling there is something wrong in the front area - I think I'll check where in the spine the front elements sit, and if altering that makes a difference. I'll report back as time allows.
04-08-2012, 04:51 PM   #6
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Just a good news follow-up. This afternoon I opened up the front, which turned out to be very easy on this lens, and used an image of the lens grouping from Bojidar's excellent site to compare the lens diagram with what I was seeing. Turned out that the fourth element had been reversed on my copy! I imagine that someone had cleaned it before, maybe to sell it, as there is very little dust inside the lens, but had reassembled it incorrectly, quite easy to do, I guess. Weather was not so fine today, but I'll take some pics with it now to see what they look like when there is better light. Thanks again TomB_tx and ripit, for responding to my posting.
04-08-2012, 07:43 PM   #7
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Not to hijack the thread, but... A couple days ago I received a Vivitar-Komine Series 1 Version 3 70-210/2.8-4 in PK-A mount. Today I took it out for its first walkabout workout, with CIF on my K20D. I notice that even in the f/2.8-5.6 range, CIF sometimes doesn't want to trigger on distinct, contrasty, well-lit distant subjects, as though the lens has difficulty focusing to infinity. I saw this with a few test shots yesterday too. I've not had this happen with my other similar mid-long MF zooms by Kiron, Samyang, Tokina. I need to take it out for more sessions to see if it's truly failing to reach infinity with the aperture wide-open. Has anyone else experienced and fixed this?
04-09-2012, 11:53 AM   #8
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RioRico - may be best to start a new thread, because I've resolved my problem, but you're welcome to carry on the conversation here - wonder if it's possible to edit the title, like the sales section? Maybe to a more general Focus Problems thread - I'll try!

If there is an infinity focus problem with the lens itself, I can't imagine that apertures size will have any effect, and it may indicate a mechanical or optical problem, but if you say "sometimes doesn't want to trigger", it may indicate that precise focus is actually not occurring every time. Is it systematic, maybe there's some front or back-focusing going on? Would it be useful to test it on Yvon Bourque's Pentax K20D AF Adjustment Chart-3 to establish accuracy?

04-09-2012, 01:39 PM   #9
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QuoteOriginally posted by demondata Quote
wonder if it's possible to edit the title, like the sales section? Maybe to a more general Focus Problems thread - I'll try!
That should be possible. I think I've done that before. Or I may be hallucinating.
QuoteQuote:
Is it systematic, maybe there's some front or back-focusing going on? Would it be useful to test it on Yvon Bourque's Pentax K20D AF Adjustment Chart-3 to establish accuracy?
I'm not sure about BF/FF issues with MF lenses. But I'll check that adjustment chart, and try another couple sessions to see if it's a recurring problem. Thanks!
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