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06-07-2019, 08:04 AM   #1
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Pentax K SMC-M 35mm f2.8 focus ring stiff

Hi guys, just got myself the above lens from a reasonably good seller in Cambridge, UK. They've reportedly had all vintage lenses dismantled and examined.

What I find odd is that the focusing ring on this lens gets progressively more stiff as you go from 0.40m to 0.30m. It occasionally gets a bit less stiff, it's not bumpy, and you can just about convince yourself it feels a bit grainy.

You know when you screw on a cap on a bottle and you give that last twist? Exactly like that.

I prefer NOT to replace this lens as it's quite rare to obtain and I got it for 40 when it's more like 70 on eBay.

Anyone have experience with these lenses? Do they do this normally? Any way I can take the back plate off and oil the mech myself?

06-07-2019, 08:51 AM   #2
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Hi there. Welcome to Pentax Forums!

In my experience with numerous Pentax-M lenses, there should be no question at all concerning the manual focus action. Pentax-M lenses typically exhibit very smooth action across the focus range, with no 'stiffness' or binding at any point. There certainly should be no 'grainy' or gritty feel. Think of the focus action as a thick knife cutting through soft butter.

Although most of the Pentax-M lenses I've handled or cleaned have had a proper focus feel -- Pentax made their manual focus lenses well back in their day -- I think one of them required disassembly, a thorough cleaning, and re-greasing of the helicoid threads.

Removing the mounting plate is relatively easy, although the screws may be very tight -- stripping out the screw heads is a possibility. Regardless, a quick oiling probably won't suffice. Furthermore, oil may seep to places it shouldn't be, especially the aperture blades.

- Craig
06-07-2019, 09:02 AM   #3
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QuoteOriginally posted by Wbl24cam Quote
Hi guys, just got myself the above lens from a reasonably good seller in Cambridge, UK. They've reportedly had all vintage lenses dismantled and examined.

What I find odd is that the focusing ring on this lens gets progressively more stiff as you go from 0.40m to 0.30m. It occasionally gets a bit less stiff, it's not bumpy, and you can just about convince yourself it feels a bit grainy.

You know when you screw on a cap on a bottle and you give that last twist? Exactly like that.

I prefer NOT to replace this lens as it's quite rare to obtain and I got it for 40 when it's more like 70 on eBay.

Anyone have experience with these lenses? Do they do this normally? Any way I can take the back plate off and oil the mech myself?
Mine does not exhibit that stiffness. The m lenses are easy to disassemble but you don't want to use oil. You need to use a grease on the helicoil after cleaning off whatever is causing the problem. Hopefully someone with more experience will point you in the right direction as to what grease to use. you do not want it liquefying and running all over the aperture blades.

Also get yourself a set of JIS screwdrivers. Normal Phillips type heads should not be used.
06-07-2019, 09:15 AM   #4
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Sounds like the grease on the focusing helical has dried out and accumulated at one end. One of my Minolta lenses suffered from this. Disassembly, cleaning and re-greasing is the solution if this is the case. It is possible the lens barrel suffered some impact damage and part of the barrel is dented or off round.

If the seller allegedly takes the time to disassemble and inspect the lenses you would think they would take the time to re-grease the lens too where needed.

06-07-2019, 09:47 AM   #5
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Many thanks all!

It's a shame that it seems that the sellers haven't had a good look at the lens mechanically. I've got a set of glasses screwdrivers and I'm planning on getting some lithium grease for the helicoid, unless someone has any suggestions as to what to get?
06-07-2019, 10:10 AM   #6
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QuoteOriginally posted by Wbl24cam Quote
It's a shame that it seems that the sellers haven't had a good look at the lens mechanically. I've got a set of glasses screwdrivers and I'm planning on getting some lithium grease for the helicoid, unless someone has any suggestions as to what to get?
The screws are JIS standard and use of other than JIS screwdrivers is done at the risk of stripping the heads. Relubing may require other specialized tools, depending on the lens. As for grease, the grade and type is important due to the tight tolerance of the threads. I use something called MicroLubrol Helimax-XP, made specifically for optical and instrumentation helicoids. A 28 gm tub is about $13 USD. You will also need naphtha or similar for cleaning out the old grease.

Are you sure you don't want to work with the seller? It is quite possible that they did not properly reassemble the lens, with the binding being the result.


Steve

Last edited by stevebrot; 06-07-2019 at 01:18 PM.
06-10-2019, 01:59 AM   #7
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Hi all, many thanks for your responses so far.

So I took the lens to a party, where a friend noted a possible dent to the front of the lens. In addition playing with the lens in more detail it actually gets a bit stiff around 0.40 - 0.38m and then lightens up again.

I feel this is likely due to a bump in the lens' history. The issue doesn't bother me that much, and ironically the aperture ring is smooth with this copy. Optically it is beautiful.

I love this thing, and will just leave it as it is. But thanks for your measured response guys, and not just saying "ditch it and get a new one"
06-10-2019, 06:19 AM   #8
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QuoteOriginally posted by Wbl24cam Quote
and not just saying "ditch it and get a new one"
There are very few things that could make a lens not worth saving. These older lenses especially are great candidates for repair given you have the time and the patience.

06-10-2019, 06:31 AM   #9
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The most common problem with that lens is sticky aperture blades but I'm happy to say mine doesn't suffer from any problems and I've never heard of that model exhibiting the problem you describe.

It's most likely in need of re-lubrication but could be a bump somewhere has knocked something out of shape.
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