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06-15-2009, 12:09 PM   #1
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Oily old lens-repair help requested

Hello all! (My first post)

I recently purchased an old Pentax A-series 35-70mm one-touch zoom Pentax Lens Review Database - 35-70mm F3.5-4.5 (Internal link). I bought it second-hand in an antique shop, not a photography store. The internal optics have visually-apparant bubbles of oil on them, and the resultant pictures are hazy, although not from poor focus (at least I think): the haze appears to be constant in each picture (clearly distinguishable patterns, etc.). I would guess that the previous owner left the lens in an auto in the sun. I am interested in repairing the lens myself.
I have some experience in optics, although not on camera lenses. I do not have many specialty tools. Any advice in addition to tips on dismantling, cleaning procedure, and re-assembly would be greatly appreciated.

06-15-2009, 12:36 PM   #2
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A quick google search turned up this very interesting result.

Markus Keinaths Photohomepage - Camera And Lens DIY, Repair, Modification

You might find your answer there.
06-15-2009, 01:30 PM   #3
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I have most of one of these lenses, bought cheap as a parts lens. From the back, work in a clean environment where small parts can't escape. The mount has an electrical contact for the A position. When the mount is removed, the aperture ring can slide off, allowing a spring-loaded ball bearing and its spring to fly free. The A button on the ring is also loose. Make sure you control all these loose parts or you won't be happy with your repair. I would not remove the aperture ring, maybe even tape it on so it doesn't slide off while you're not paying attention.

There is one lens element within the mount, unusual for Pentax. I was able to unscrew the group of three more rear elements by zooming so they protruded and getting a good grip. The group also has spanner slots as described below. With this group removed and the aperture opened, you may be able to clean the back of the front lens group. Optical diagram:



If oil is the extent of your problem, the oil probably didn't penetrate inside the groups. I would start from the rear and clean all the surfaces you can access, hopefully get the rear of the front group clean, and see if the trouble is fixed.

There was a beveled ring in front, inside the filter threads. I can't remember if it was glued in or unscrews via the filter threads; I think glued in. Removing that gives access to what's holding in the front elements. You'll need something to fit in slots like the ones marked with green arrows here:



On my lens, there are two concentric rings. Removing the outer one is impossible. Removing the inner one does nothing. So I'm not much help here.

If you have a reason to take the lens apart more, there are screws hidden beneath the rubber focus grip and also hidden by the aperture ring.
06-17-2009, 11:28 AM   #4
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Thank you very much! I had planned on going in through the front based on the exploded diagram... thanks for the tip!

06-22-2009, 01:59 PM   #5
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-Have completed repairs. Oil was in between the center and front elements, obviously from the zoom mechanism. Cleaned up easily with Murphy's oil soap and LOTS of water. Element remained sealed. Easily accessed via the front. Remove the beveled ring so the front looks like it does in the above photos, and then used two screwdrivers (flathead) to tease out the front element. Do not remove front element by the initially exposed slots, as you can slip onto the lens surface. Removed what is I think the outer ring in your diagram, the one you said was impossible to remove. Not impossible, just very difficult.

As I said, it is helpful to tease this ring off before attempting to remove the front element.

General words to beginner camera repairmen/women: Do it! it isn't as hard as it looks so long as you go slowly and keep track of parts. it's worth it!
Attached Images
 

Last edited by Mudder; 06-22-2009 at 02:00 PM. Reason: sorry about duplicate images.
06-22-2009, 02:05 PM   #6
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QuoteOriginally posted by Just1MoreDave Quote
If you have a reason to take the lens apart more, there are screws hidden beneath the rubber focus grip and also hidden by the aperture ring.
Dave.... I swear you've probably opened up all pentax lenses and took pictures of the insides of each one...
06-22-2009, 03:29 PM   #7
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QuoteOriginally posted by Andi Lo Quote
Dave.... I swear you've probably opened up all pentax lenses and took pictures of the insides of each one...
It's probably not for everyone, but I like taking something that's worthless as it is, and making it useful again. And the pictures are a good excuse to use my macro lenses.

Camera repair is not as much fun. Too many parts.
06-23-2009, 06:41 AM   #8
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Nice job on the repair.

I have a 50/f4 Macro Tak that I'm afraid I'll have to open up one day as the ap blades are sticky some days. I might have to pick Dave's brain on that one if I can't find any info out there. AS long as the blades continue to eventually get to the right spots and don't deposit any oil on a lens, I'll be leaving well enough alone.

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