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01-20-2015, 12:59 AM   #1
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SMC Pentax 35mm f3.5 - Sluggish and sticky diaphragm especially when cold

A few months ago, I bought a used SMC Pentax K 35mm f/3.5. I have no trouble using it indoor and am pleased with its image quality. It performance is consistent with the reviews and feedback I have read in this forum.

However, I notice that every time I use it outside, where the temperature ranges between 10 and 1 degree C, the lens diaphragm becomes sluggish leading to consistently overexposed pictures. I have no such experience with my other K, M, A, and DA lenses when used in cold conditions.

Does anyone have the same experience with this SMC Pentax K 35mm f/3.5 lens?
Could this be related to the lubricant in this old lens? Does anyone have any experience dismantling and cleaning this lens?

01-20-2015, 02:45 AM   #2
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Firstly, common with the late Super Taks, and SMC Taks, and since the K's were just SMC Taks in K mount, makes sense that you are experiencing it too.

As for taking them apart, just give it a crack! If you can use a screw driver you can get it sorted. There are countless videos, if you cant find one about a K, find a M, or A or even a Super Tak. Structural they are all familiar.

And yes my 35mm F3.5 SMC Tak behaves like yours, but I have the advantage of being able to stop it down manually and then shoot, thus not worrying about the blades getting into position in time.

If it's all to scary, find a decent camera shop, they should be able to direct you to someone who repairs lenses. They need not be a Pentax specialist.

Last edited by tromboads; 01-20-2015 at 03:52 AM.
01-20-2015, 03:45 AM   #3
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Many years ago I had the same problem with an old east-German lens. But it allowed me to apply two tiny drops of break oil right in the plane of the blades by unmounting the aperture ring. And it helped. Break oil dissolves the remainings of old greas.
01-20-2015, 07:04 AM   #4
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I can confirm the problem with east-German lenses. The oil solidifies with time, especially if the lens is not used. I got an aesthetically mint Pentacon 200 f4 Preset a while ago - but because it was unused it had this problem. You need to open the lens, remove as well as possible the solidified oil and put some new one.

I'm not sure about the aperture though - the aperture blades should be clean from oil - when oil gets on them it is not a good thing at all, especially if you want to sell a lens like that. So probably oil got on the blades, it was unused for a long time and it had time to solidify there. Because of this oil-on-blades problem of yours perhaps is a good idea to bring it to a lens repair shop.

01-20-2015, 01:58 PM   #5
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You do not use any oily lube on the blades of an aperture ring. If there is grease it leaked there from somewhere else in the lens, usually the helicoid grease when the lens got hot. Then it seems to want to travel on the edges of the blades to the middle and make the thing stick. Cleaning it is more of an access challenge than anything else. Some lenses have loctite on the screws and just don't want to come out. For sure use precision JIS screwdrivers to avoid camming out and stripping the heads.

To get the old grease off, I try to access the aperture from both sides by removing optics in the way. Then gently rub it with a Q tip with solvent, which gets most of it. If its deep in, I use medical swabs, basically long Q Tips. Do both sides, over and over. You can use powdered graphite dissolved in alcohol as a paint on dry lube, but very sparingly. This works 90% of the time to free the blades.

Otherwise you have to remove and disassemble the aperture ring. You clean the blades in an ultrasonic cleaner and re-assemble. They are fragile and easily bent, so this demands great care. Once you get it back together, inserting it can be tricky. I use double sided tape on the end of a rod to drop it back in. Sometimes I tighten the springs to make it snap, because they seem weak after decades. Every aperture is a little different so this is just general advice.
01-20-2015, 02:05 PM   #6
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Avoid a lot of headache and send it to Eric Hendrickson...

01-20-2015, 03:17 PM   #7
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Just to mention again: the blades are meant to work with no oil at all. Oil just slows them down, no matter how light the oil is. The Pentax lens designs usually keep oil or grease separated from the blades. I think the blades are not the problem here, if they work OK at normal temperatures.

Some lenses do have a pivot point for the aperture lever or a long, complex mechanism (macros). These might benefit from a small drop of oil. You'd have to take the mount off (5 JIS screws) and see what the aperture lever is doing. Then use the smallest amount possible and see if that helps.

If you don't already have the right screwdrivers, a lens repair guy makes sense. The lens is 40 years old and the manual says it should be checked out once a year. You might find someone in Canada.
01-20-2015, 03:29 PM   #8
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QuoteOriginally posted by HYS Quote
apply two tiny drops of break oil right in the plane of the blades
No...not on any auto-aperture lens...

QuoteOriginally posted by Just1MoreDave Quote
Just to mention again: the blades are meant to work with no oil at all. Oil just slows them down, no matter how light the oil is.
What he said. The iris diaphragm on an automatic aperture lens is designed to work without oil. If oil does migrate onto the blades it is a sign of trouble and, if found on the blades of a used lens, is reason to avoid purchase or return.

There used to be a lens repairman on The Island (Vancouver Island, that is) who worked on Pentax. I don't remember his name, but I believe he used to be active on this site.


01-20-2015, 11:03 PM   #9
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Thanks everyone. I've never repaired a lens but I have enough tips from you guys to give it a try with confidence knowing that it is doable .

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