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07-30-2013, 05:12 AM   #1
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Pentax M 28 / 2.8


today I got a cheap Pentax M 28 / 2.8. I got it cheap, because the aperture blades are closing a bit slowly. I took the risk to buy it because I think it can be repaired. I once repaired an old Tokina RMC 28 / 2.8, which was not that difficult. Does somebody know, if the Pentax M is similar to the Tokina with respect to dismantling the front and rear lens elements in order to clean the aperture blades?



07-30-2013, 05:28 AM   #2
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I have one (7162082) with same problem. It works better when warmed up so it is probably a dry lubricant problem. Aperture blades are visible and they are dry and clean.
Can you take off the mount first, to see if there is a pivot in there you may be able to put a drop of solvent on?
I will be interested to follow your progress, however i use a Vivitar 28/2 mostly.
07-30-2013, 06:57 AM   #3
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JimKnopf, quick test also if you have a chance... Gently and while the lens is set to infinity, tap the front against a not too hard surface... a book, or sheet of cardboard. Then check to see if any change in blade action. My M28/2.8 seemed the same slow blade problem and dry lube didn't solve. When I took the lens apart is when I found that the set screw broke loose and allowed the front portion of the aperture assembly to press down too tight on the blades in the lower cup portion. Since servicing the lens and re-set and thread-locking the screws, no issues since.
07-30-2013, 08:15 AM   #4
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@wombat2go: I'll keep you posted ...

@MysteryOnion: What helped with my Tokina was to really get out the lens elements and then putting some acetone on the aperture blades. Basically this would be the first thing that I would try. However, before opening up the whole thing, I would still need to try the lens out more thoroughly, as I need to analyze under which conditions I get overexposed images.

Last edited by JimKnopf; 07-30-2013 at 08:54 AM.
07-30-2013, 08:19 AM   #5
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The aperture blades are meant to be run dry. Acetone will certainly clean the blades of oil but it should not be applied in situ. The aperture assembly should be taken apart and then cleaned. Acetone has a nasty habit of attacking any plastic parts, paint, etc.

I have no experience with the Tokina lens, so I can't help you there. However, the M28/2.8 is fairly easy to take apart and it's more or less the same procedure as disassembling the M50/1.4 or similar. There are plenty of articles on this forum and the web on how to do it.

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