[pc123]

I have a SMC Pentax-M 28mm f2.8 which is mechanically good but riddled with fungus. I've managed to get the front apart and roughly cleaned the front elements. I will need to re-do this properly. I'm stuck at dissembling the rear elements. It appears there are two variants of this lens:

SMC Pentax-M 28mm F2.8 Reviews - M Prime Lenses - Pentax Lens Reviews & Lens Database

I've searched on line for this lens and similar, and of course, I can only find information on people disassembling the other variant. On the other variant you can simply put a lens spanner on the rear lens assembly to unscrew it. However, on mine that does not work. There appears to be a ring you can unscrew, but it is hidden under a wider flange that blocks a regular lens spanner. Pictures attached.

Has anyone any experience with this type of assembly? Before I do something that potentially causes damage I thought I would ask? As I see it my options are:

1. Grab the sharp flange with a rubber glove and apply an unreasonable amount of force (this will hurt, the flange is sharp!)
2. Place the tip of a small screwdriver into the notch in the ring just below the flange and gently tap it. This will likely cause some minor damage to the notch.
3. Less favoured, drill a couple of small holes in the flange, or file a couple of notches and try a lens spanner on it.

Thoughts?

Incidentally, I bought some JIS screwdrivers to remove the bayonet mount. I'm very surprised that I did not strip the screws, snap their heads off or destroy the screwdriver bit. It took a lot of force to undo them.

Attached Images

   

[Henrico]

I have a SMC Pentax-M 28mm f2.8 which is mechanically good but riddled with fungus. I've managed to get the front apart and roughly cleaned the front elements. I will need to re-do this properly. I'm stuck at dissembling the rear elements. It appears there are two variants of this lens:

SMC Pentax-M 28mm F2.8 Reviews - M Prime Lenses - Pentax Lens Reviews & Lens Database

I've searched on line for this lens and similar, and of course, I can only find information on people disassembling the other variant. On the other variant you can simply put a lens spanner on the rear lens assembly to unscrew it. However, on mine that does not work. There appears to be a ring you can unscrew, but it is hidden under a wider flange that blocks a regular lens spanner. Pictures attached.

Has anyone any experience with this type of assembly? Before I do something that potentially causes damage I thought I would ask? As I see it my options are:

1. Grab the sharp flange with a rubber glove and apply an unreasonable amount of force (this will hurt, the flange is sharp!)
2. Place the tip of a small screwdriver into the notch in the ring just below the flange and gently tap it. This will likely cause some minor damage to the notch.
3. Less favoured, drill a couple of small holes in the flange, or file a couple of notches and try a lens spanner on it.

Thoughts?

Incidentally, I bought some JIS screwdrivers to remove the bayonet mount. I'm very surprised that I did not strip the screws, snap their heads off or destroy the screwdriver bit. It took a lot of force to undo them.

That flange will have a screw connection also, by turning it contra clock wise, it doesn't loose?

[pc123]

That flange will have a screw connection also, by turning it contra clock wise, it doesn't loose?

Thank you. I used a bit more force on it. A latex glove did not give me enough grip but bare fingers did. After using quite a lot of force that top ring came unscrewed. It retains the rear element. Once it was removed then I could use a lens spanner to remove the rear assembly. Once the rear assembly was out I found a couple of holes for a lens spanner on its forward face. So hopefully I can dissemble this group and clean them. That is something for later. I unfortunately marked the assembly as the lens spanner slipped, but I am only learning and you have to start somewhere. One reassembled that will not be seen . So a bit more care needed in adjusting lens spanners. But if it cleans up well I got it for a good price - but not free, so I hope I can clean it up.

[Henrico]

The lens elements in the M28 are very small. Cleaning them with a cotton handkerchief and pure ethanol often does the job. Cleaned this is a fine lens on APS-C. Compact and pretty sharp stopped down a bit. Good luck!

[pc123]

The lens elements in the M28 are very small. Cleaning them with a cotton handkerchief and pure ethanol often does the job. Cleaned this is a fine lens on APS-C. Compact and pretty sharp stopped down a bit. Good luck!

I've got 99% isopropyl alcohol, hydrogen peroxide and also some lens wipes. This is the first lens were I've had to strip it down so far. Thanks for wishing me luck.

[Not a Number]

I wouldn't recommend anything stronger than 50% alcohol to clean lenses. I've seen 99% isopropyl alcohol strip the coating and black anti-reflection coat on the edges off lenses.

[pc123]

I wouldn't recommend anything stronger than 50% alcohol to clean lenses. I've seen 99% isopropyl alcohol strip the coating and black anti-reflection coat on the edges off lenses.

Thanks. I can dilute.

[Henrico]

Always good to start carefully.

I have some experiences with SMC coatings. I once cleaned an A400/5.6 with pure ethanol and isopropanol and that went fine, coatings were intact, fungus gone, elements cristal clear and lens IQ is perfect.

I also had a K24/2.8 that suffered balsem separation. I tried everything to split the two glued elements, these swam in aceton, ethanol, thinner, hexane but nothing worked. Finally I heated the elements in sunflower oil up to 160 °C, I had read somewhere on the internet that should work but no, no result. These elements were obvious glued with something different than balsam. Finally I cleaned the twin elements in ethanol and the blue/purple SMC-coatings were still there as before, incredible. I never experienced Pentax SMC coatings that could swiped of easily with solvants.

But take care and be aware of the risk of harming your glass...

[pc123]

Always good to start carefully.

I have some experiences with SMC coatings. I once cleaned an A400/5.6 with pure ethanol and isopropanol and that went fine, coatings were intact, fungus gone, elements cristal clear and lens IQ is perfect.

I also had a K24/2.8 that suffered balsem separation. I tried everything to split the two glued elements, these swam in aceton, ethanol, thinner, hexane but nothing worked. Finally I heated the elements in sunflower oil up to 160 °C, I had read somewhere on the internet that should work but no, no result. These elements were obvious glued with something different than balsam. Finally I cleaned the twin elements in ethanol and the blue/purple SMC-coatings were still there as before, incredible. I never experienced Pentax SMC coatings that could swiped of easily with solvants.

But take care and be aware of the risk of harming your glass...

Thanks. One thing I've noticed, or not noticed - blank paint on the edged of the glass so far (so far being important). I don't think anyone else has had ti apart. Is black paint normal - of does it just vary from lens to lens.

[Henrico]

The black paint is to reduce internal scattering in the glass itself. That paint does not survive solvents. I used a black permanent marker pen to fix that.

[pc123]

The black paint is to reduce internal scattering in the glass itself. That paint does not survive solvents. I used a black permanent marker pen to fix that.

What if it does not appear to be there in the first place? I assume that the lens has not been opened previously? If I put permanent black marker on there I don't think it would do harm and could be removed anyway.

[Henrico]

If it not there factory wise, light beams will not touch the glass borders, you can leave it as it is.

[pc123]

If it not there factory wise, light beams will not touch the glass borders, you can leave it as it is.

Thanks.

[pc123]

Thank you to everyone. Finally got around to disassembling and cleaning it. A few apprentice marks unfortunately, but when reassembled they do not notice. If I'd not mentioned it here no one would know. I probably will re-do it as there is quite a bit of dust inside now. But the fungus is gone, the most important part. There might be a small amount of marking on the front element, but it is still 100% better than it was. One learning thing was that lens wipes, the impregnated sort, seem fine, but if you reassemble the lens shortly after using the wipes residual moisture off the wipes gets trapped in the lens and causes haze. I've ordered some lens microfibre cloths to use as a final polish rather than the lens wipes to see if that does the trick.

[pc123]

Thank you to everyone. Finally got around to disassembling and cleaning it. A few apprentice marks unfortunately, but when reassembled they do not notice. If I'd not mentioned it here no one would know. I probably will re-do it as there is quite a bit of dust inside now. But the fungus is gone, the most important part. There might be a small amount of marking on the front element, but it is still 100% better than it was. One learning thing was that lens wipes, the impregnated sort, seem fine, but if you reassemble the lens shortly after using the wipes residual moisture off the wipes gets trapped in the lens and causes haze. I've ordered some lens microfibre cloths to use as a final polish rather than the lens wipes to see if that does the trick.

Just a quick update to finish off this topic. Following the first cleaning attempt the lens was fungus free albeit I had introduced a fair amount of dust. In use however I was not having much luck with the lens. Images were just below average. Not quite sharp like I was always missing focus. They would sharpen up surprisingly well in post, but were not satisfying straight out the camera unlike the rest of my lenses. Apart from the dust I could see no issue. I disassembled the lens and polished each* element by breathing on it and wiping it with a new microfibre lens cloth and reassembled. This worked, the results were might and day and I'm now happy with the results of this lens straight out of camera.


* there was an assembly of two elements I could not easily separate, but they were fungus free inside so I did not make the attempt to try separating this pair.