Minolta M-Rokkor 28/2.8 spotting corrosion - Advice needed

[ForenSeil]

I have bought an Minolta M-Rokkor 28/2.8 in Leica-M Mount.
What a beauty! Build quality is almost Leitz-like.
No Schneideritis, condition is VERY good, except some spotting corrosion on the rearside of the front element.
According to some guys on the web it may damage the glass if it's not removed.

According to this thread (third post) it's easy to repair and it costs about 75€
http://www.l-camera-forum.com/leica-forum/leica-collectors-historica/241026-minolta-m-rokkor-dusty-second-element.html
Here's one service which offers repair for this, together with shipment it would cost about 100€ effectivly.
http://www.kamera-service.info/index.php/de/

But I wonder if the front element can be easily removed buy unsrewing the ring and cleaned without disassembling of the whole lens by an amateur like me. I have an spanner tool which could be easily modified to fit into the wholes seen pic #3
Would you risk to try it?








(on this pic it looks worse than it actually is)[list=][/list]

[kuuan]

great buy, super bargain!
Just unscrewing the name ring should allow you to take off the front glass, if the damage, need of cleaning only is there it should be very easy to do

[ForenSeil]

My spanner wrench had to be sharpened to fit into the tiny holes. Luckily my spanner wrench is made of steel and I have a very good 120 sharpening stone












DEAD END. Anything in front of the aperture seems to be glued into one block. Any Idea how to go further?

[hoanpham]

i never have this lens, but my rokkor 35/2.8 has this kind of solid block.
when i look closely from the front, there might be possible to open up the largest element.

the inner glass elements fit inside the metal house, smallest at bottom, then close all by the largest.

i dont think it affects the image, but annoying to look at.

[kuuan]

I'd simply try if there aren't parts that can be screwed apart, not using the spanner but the fingers and maybe some cloth or alike to get a better grip.

[ForenSeil]

I already tried that - no chance, nothing is moving.

What do you think - keeping as is, reselling or repair (75€+shipment)?
Is it worth a try if you already have a ZM Biogon 35/2?

[DR.JUAN]

http://www.rangefinderforum.com/forums/showthread.php?t=115677

Good luck.

[kds315*]

I had that lens and similar spots. Used it, was great, did not get worse over the years.
Some people just bugs that kind of stuff and they need to get "rid of it" no matter what.

I for my side know that I'm human and have a lot of flaws, still "work pretty well", so
I give that same credit to the stuff I use... (as long as it works)

[kuuan]

ForenSeil may I ask, have you done somthing about the lens, did you succeed in repairing it or did you have it repaired?
I also got this lens. It has both Schneideritis and spotting corrosion to a high degree. IQ certainly suffers and it needs repair.
thank you, andreas

[kuuan]

mine is a quite severe case, besides the bigger white points around the rim there also are small white points throughout the front lens.

Photos suffer from rather strong ghosting, specially at wider apertures, only @f8 it's getting acceptable. My lens certainly needs repair!

[itsdoable]

I just stumbled on this thread as I was looking for ideas on how to dismantle the front cell. As the last post was only a month and a half old, this might be useful. I have not dismantled the front cell yet, I need to find a proper fine saw.

If the front element is a doublet, you should see 3 (or 4) reflections, a small flash light in a dark room will tell you pretty quickly. I only see 3 elements in 2 groups in the front cell.

The white spots (Schneideritis) is under the black flocking paint around the edge of the element, the result of contamination on the glass and oxidation of the paint. You have to remove the paint (without affecting the AR coating) to remove the white spots (I have not done that yet, so I'm not sure what solvents to use, but you can also sand it off). Once you have done that, check to see if you still have spots in the center, they may be reflections of the spots on the edge.

My problem is condensate on the inside of the front element. It only has a few mild white spots.

[kuuan]

Hi and welcome to the forum.

Removing the paint is very easy, I just scratched it off. With the paint removed it was easy to see that the front "element' in deed only is a single element, not a cemented group of two. without the paint the glass still shows spots because of etching, but if repainted propperly ( which is not easy and I have not done yet, only tried parts ) the white points aroud the rim should have disapeared.
The small spots throughtout the middle of the lens are very difficult to come by though! They are all etchings of the glass resp. possibly only it's coating. I have polished quite some hours by now, trying various glass and metal polishers, which made the lens, if looked at it from the front, look clean by now, but if looked at it from the back and with flashlight shining through one can still see them quite prominentely. Performance is improved but unfortunately still far from good, not sharp, worse at wider apertures.
So far i have only polished by hand. I will try again, most likely with cerium oxide, once I have access to a dremel or similar machine

[itsdoable]

A little cruder than the above, but I did not have as fine a saw.





A close inspection after cutting off the ring shows that it really did not have to be cut off, as the part that was removed does not hold the element (but it does make it a lot easier to access). The lens appears to be glued in, or has a very tight fit, but the aluminium sleeve is open at both ends, and the front element and rear doublet are simply pushed in and fixed buy a cement.

Question for Kuuan: how did you pull the element out of the sleeve, the suction cups I have cannot pull it out, and prying has not budged it.

[kuuan]

[animaux]

@itsdoable Did you succeed in removing the front element?

I was not able to remove it at all, even after sawing off the notorious ring too. I suspect my front element was opened before and somebody glued it back into place.

I had sent a macro image of the haze on the back of my front element to www.kamera-service.info in the Netherlands. They replied »It looks like the front element has severe ‘pitting corrosion’. That cannot be helped/repaired. And indeed, there are no longer spare lens units available. (…) In this case, we think it is not worthwhile to do something about it.«

That left me less frightened about using more drastic measures :) especially since the coating on the outside of the front element was severely damaged since i bought it as well.

In the end I widened the hole in the side to about 5mm with a drill. This way I was able to access the inside and clean it with pure alcohol. The result is quite good. Effects are hardly visible in the images anymore. So it might not have been that severe case of pitting corrosion after all. I still need to cover the newly exposed aluminum with matte black paint though.

[itsdoable]

I should post an update, clearly I don't visit here often enough.

There was no way I could get the front element out of that cell with a suction cup device (I tried a few), although in retrospect, if I had a custom jig and cup designed for this specific lens cell, and maybe some knowledge of the bond used on the element (both front and rear elements are bonded into the aluminium tube, the ring really did not have to be cut off), maybe I could have gotten it off.

So I went the next step, I drilled down past the front element so I could pry out the element from below. It took a bit of force before it popped loose, and then I could work it out. In the process, I chipped the edge of the element (and in retrospect, I believe I can now repeat this without chipping the lens).



The chip on the edge does not extend into the optical surface, so despite the ugliness, it's optically still functional.

The inside of this element was covered with tiny droplets of liquid that was slightly caustic to the touch - I wish I had taken a proper picture of that before I wiped them off. In the center of each droplet was a tiny pit or white deposit. I was hoping that it was a deposit, but that does not seem to be the case, I tried various cleaning and reducing agents (they resembled calcium/oxide like deposits) but I'm not sure they had much effect, they may have reduced the spots a bit. This is the best close up I have of what the pits look like after 2 cleanings:



This is what the element looks like after the lens was reassembled, you can see the chip on the lower left edge where my blackening did not completely hide it. You can see some of the pits on the inside of the front element.



Here is a sample image taken with the lens, it has slightly reduced contrast, but still much better than it was prior to the cleaning.



@animaux: My front cell did not have a hole in the side. Maybe that is why the liquid droplets remained. I'm sure that the liquid would eventually evaporate as leave the circular pits that ForenSeil and others have. Of course my cell now has 2 holes...

[kuuan]

finally got some "Cerium Oxide', polished the effected surface and successfully removed the white spots and repainted the black, outer rim.



unlike comments found on the net my impression had been that the lens, with the white spots, lacked sharpness and I believe that now, after removing the spots, generally both sharpness and contrast are improved.
However removing the white spots most likely actually was removing the coating from the rear of the front lens that had gone bad.
As a result, first test photos show, now it easily blooms around highlights, specially when used against light.




a crop of the 'sensitive' area:


The blooming certainly effects general usefulness of the lens, but I also like the effect and will try to use it.

another trial shot 'not' against light

Untitled by Andreas, on Flickr

[kds315*]

Well CeO2 polishing obviously also removed the coating, expectedly. No wonder you have that effect now.

[kuuan]

[kuuan]

repainted the outer rim once more, that improved the lens a bit further, blooming still there but reduced, I call it "leicable glow"

Untitled by Andreas, on Flickr

Untitled by Andreas, on Flickr

Untitled by Andreas, on Flickr

Untitled by Andreas, on Flickr

[ForenSeil]

I've never seen such a strong glow caused by bad coating.
Maybe front- and rear elements are not 100% parallel?

Btw., you can also use chromium oxide (ingriedient in the paste you can buy for stropping straight razors or as pigment for oil-based dyes, naturally ~0,3-1µm) or better diamond lapping spray/paste (0,5µm/0,25µm) instead of Cer(IV)oxide (~1µm), both are finer and diamond is by far most effective

(Sidenote, I've tried many lapping sprays, powders and pastes for extreme sharpening of hardened high-quality carbon steels - avoid cheap diamond lapping pastes from Ebay like HK-Tools etc., they are crap which could harm your lens(!), but a "quite cheap" source of highest quality diamond paste+spray is Abradia-PS.com(for Europe only) )

[kuuan]

thank you very much for all the info regarding polish!

[ForenSeil]

[kuuan]

thank you very much "Forenseil' to come back to answer.

coming back to this thread and looking at my own photos of the lens when freshly taken apart I noticed that I had forgotten to repaint one of the surfaces! Immediately I did that, and now it seems to glow less again
Misalignement seems not to be the problem, the fit in it's housing is very, very tight.

here some latest samples:

Dray Nur waterfall by Andreas, on Flickr

a similar photo I also had taken the the M-Rokkor 2/40, and the 40mm handled the highlights of the reflecting sun on the water quite a bit better

Untitled by Andreas, on Flickr

but that also was quite an extreme situation and generally I like the photos it gives me, just I must be aware of some limitations.
Not 100% sure that this was the 28mm, could have been the 40mm (?), if with the 28mm then this is probably one of my better pics with it where it proves to be behaving quite well

Dray Nur waterfall by Andreas, on Flickr

cheers, andreas

[kuuan]

of this one I know for sure that it was taken with the 2.8/28

Khai Doan Pagoda by Andreas, on Flickr