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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...