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Minolta M-Rokkor 28/2.8 spotting corrosion - Advice needed
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PostPosted: Tue Jul 18, 2017 12:12 am    Post subject: Reply with quote

Thanks to all who contributed to this thread. I just want to confirm that you do you need to cut off the metal lip on the front element I tried for hours without cutting it off and couldn't get it out as soon as I cut it off at slipped right out. Because of all the acetone are used the paint slid right off of the sides of the front element. I gave the back side of the front element a cleaning and now my lens is a stellar performer. All I did to paint the sides of the front element again was use permanent marker worked like a charm no reflections


PostPosted: Sun Oct 22, 2017 3:54 pm    Post subject: Update Reply with quote

Just another update.

2 1/2 years later, the inside surface of the front element has fogged up again. Dismantling (easy now that the bezel of the front group has been cut) and cleaning greatly improved performance.

It looks like this is going to be a regular occurrence. Some more research into the problem: it seems the doping in certain optical glasses is not stable and can corrode and/or out-gas, resulting in fogged lenses. This is kept to a minimum by specific anti-reflection coatings, which also act as a sealant. Most of the references I got for this was from early Leica glass, in which the coatings were not sufficient in quality (hardness) that these lenses degraded as they aged. With the Minolta 28mm/2.8, it seams the fog, if left on long enough, will corrode pits in the glass, which is what I have. So regular cleaning should keep this optic from degrading more (it's still functional). Or maybe it's time to look for a different lens...


PostPosted: Tue Jan 09, 2018 7:55 am    Post subject: Reply with quote

Skaterat wrote:
Thanks to all who contributed to this thread. I just want to confirm that you do you need to cut off the metal lip on the front element I tried for hours without cutting it off and couldn't get it out as soon as I cut it off at slipped right out. Because of all the acetone are used the paint slid right off of the sides of the front element. I gave the back side of the front element a cleaning and now my lens is a stellar performer. All I did to paint the sides of the front element again was use permanent marker worked like a charm no reflections


This is great information - thank you! I'm also hoping to do this. Do you have any additional tips or pictures? Did you also use a hand saw? I'm wondering if a Dremmel like tool could be used. Also do I need a suction cap, or is it possible some other way? What did you use to clean the lens? I'd be very grateful for any help Smile


PostPosted: Tue Jan 09, 2018 10:25 am    Post subject: Re: Update Reply with quote

itsdoable wrote:
Just another update.

2 1/2 years later, the inside surface of the front element has fogged up again. Dismantling (easy now that the bezel of the front group has been cut) and cleaning greatly improved performance.

It looks like this is going to be a regular occurrence. Some more research into the problem: it seems the doping in certain optical glasses is not stable and can corrode and/or out-gas, resulting in fogged lenses. This is kept to a minimum by specific anti-reflection coatings, which also act as a sealant. Most of the references I got for this was from early Leica glass, in which the coatings were not sufficient in quality (hardness) that these lenses degraded as they aged. With the Minolta 28mm/2.8, it seams the fog, if left on long enough, will corrode pits in the glass, which is what I have. So regular cleaning should keep this optic from degrading more (it's still functional). Or maybe it's time to look for a different lens...


I have serious doubts as an engineer that the glass is the reason for the fogging, it is rather outgassing of lubricants and/or paint which has been used.


PostPosted: Tue Jan 09, 2018 11:14 am    Post subject: Reply with quote

Optical glass and filters have different resistance to humidity and chemicals. A coating shiled the lens against these influences.
So it is possible, that such a thing happens. Glass corrosion has made some of my lenses unusable (before I bought them).

Regarding the effect of polishing: The lens surface form changes likely, this could be a big problem. The lens thickness is unlikely to be much problematic, cause production tolerances are likely higher.
But: In some extreme lenses the production tolerances could be compensated with for example paring other lens tolerances with it, or withdifferent thickness washers.