28mm M-Rokkor White Spots

I could use an inexpensive, small m-mount wide to replace my SLR 28mm. I'm

thinking the 28mm M-Rokkor with its common filter size with my Rokkor 40mm is

the ticket. The Pop Photo test of the 28mm M-Rokkor indicate it's an even

better performer than the 40mm. Forum members all say good things about the

image quality and size of the 28mm M-Rokkor. The big question mark is the white

spots on the edge of elements. What caused the spots and if you find a clean

28mm M-Rokkor will the spots show up in the future?

Richard: My 28mm Rokkor has no white spots, and I have owned this lens for over 15 years now.

 

Raid

Of course it is only speculation but it seems the current thinking is that if the lens has no "white spot" issue now, it isn't likely to develop them in the future. Mine was a superb performer and in my most arrogant opinion equal if not superior to the 28 Elmarit (3rd). At that though, I didn't like its size and much prefer my current CV 25 for its compactness. Nevertheless, if you use a 28 frequently the Rokkor is the best buy (again my opinion). Even one with white spots will problably perform well enough that most viewers couldn't tell the difference.

"Even one with white spots will problably [sic] perform well enough that most viewers couldn't tell the difference."

 

Probably true, but I like to try new to me Leica gear essentially for free. The 28 Rokkor and the 90 Thin TE have well known reputations. I bought a crystal clear TE (I use a Pelican Super SabreLite xenon bulb flashlight that I use on the job because it is MSHA approved for explosive atmosphere work, when inspecting lenses.). It developed the etching after a few years. I ended up giving the lens away here.

 

Avoid these lenses.

I'm not sure what lens separation looks like. Does a separation between elements create a white spot that is not completely opaque?

It is definitely not lens separation; the spots form on the rear surface of the front lens,

which is a single element. The most common explanations that makes any sense relate to

either bad coating on the rear of the front element, bad coating at the edge of the lens

(that somehow causes spots), or some kind of chemical interaction between the coating

and something else.

 

For what it's worth, my M-Rokkor 28 has what I would call a mild case of the spots that

does not seem to have any ill effect.

 

Cheers,

 

David

David, could you provide more description so I know what to look for. I understand the spots can grow until veiling glare is noticeable. Minolta was double coating the lenses they made as early as 1960. Seems strange they could make an engineering error in coatings.

I have examined many of these lenses and dont believe it to be a coating flaw. The problem is with the black paint used to coat the rim of the lens to reduce internal reflections. Basically crystals form from something leaching out of the paint. In a fair number of examples you will see the spots only evident on the rim of the front element. Because at the early stages it limits itself to the rim and not in the optical path and is why its reported it doesnt effect picture quality in alot of cases. However if the problem is allowed to continue there becomes so many crystals that they dislodge from the rim and get onto the rear of the front element where they then etch into the coating. Dan mentioned above that if the lens hasnt developed the spots its unlikely to happen, sorry but this is not so. I had one develop the spots last year. Not only that, it occurred in one outing where it was an incredibly hot day and thats when the spots appeared quite severely. This further supported the theory that the spots were crystals formed from leaching in the excessive heat.

 

Strange as the problem is, I have seen it occur in other lenses in an identical manner. One lens was the Zeiss M 15mm Hologon. So if you have a Rokkor 28 with just a few white spots round the rim my advice is to have the edges of the front element cleaned and repainted with a more suitable paint before it spreads.

Joel, excellent answer. This is the first time anyone has provided any descriptive answer on why and how the spots occur.

Actually, I deal with this problem often and these white spots are from moisture getting

under the antireflective paint and causing it to lift, creating an air bubble which appears

white. The reason it show up as a spot/dot is because the paint (just as coating) has

pores and the moisture can only reach the glass surface via the pore. The paint must be

removed and replaced but it is a big job because the front element is machined into this

conical mount in the front and a small cemented doublet (next to the aperture) is

machined into the rear smaller conical end. Minolta (with this design) created an airspace

that is difficult for the moisture to evaporate from. On the other hand condensation can

get into almost any space even under the tightest seal. This lens will often also have spots

in the coating because of moisture getting thru the coatings pores and that is even more

of a problem to repair

<p>Sorry to bring up a dead thread, but I am expecting a delivery of a CLE kit with 28/40/90 lenses with the 28 showing 'a couple of faint spots' -- does anyone know where to get this lens cleaned / repaired, and for roughly how much? </p>

<p>Cheers!</p>