Newton rings on 75mm lens for Mamiya 6. I Need advice.

[bengt_rehn]

I am thinking about to buy a Mamiya 6 equipment, but I discovered

newton rings on the 75mm lens when I hold the lens under a writing-

table lamp. The newton rings appear both on the front and rear lens

element. I have never seen this phenomena on other lenses before, the

other Mamiya lenses (50 and 150mm) was completely without rings as

well as my Canon FD lenses. What´s causing the rings? Can I do

something about it? Will they make any noticeable influence on image

quality? I will appreciate any help.

[barry schmetter]

Hmmm, of all the lens defects I've ever heard of, this isn't one of them. It sounds like it might have to do with the multicoating on the lens. I would first check to make sure the elements are absolutely clean. A thin layer of residue may be causing the problem.

 

Newton's rings are caused by interference when there is an extremely thin layer of a substance of one index of refraction located between two layers of a different index of refraction. What you may be seeing is the residue/multicoating/glass sandwich, which has a variation in refraction indexes.

 

There might also be a possibility of some separation of the elements or a problem with the optical cement, but I've never heard of this producing Newton's rings.

[john_hicks___]

I've never seen it in an M6 lens, but here's my speculation...

 

Multicolored rings are usually caused by two things, the first being element separation. This is uncommon with modern lenses but is fairly routinely seen in old lenses that use balsam cement. In a modern lens it could be caused by faulty cementing (unlikely), long-term exposure to high heat or mechanical damage.

 

Another common cause is the deposition of evaporated lubricants on inner surfaces. The lubricants are used in the helicoid and in that lens perhaps in the shutter mechanism. The lubricants will evaporate and deposit in lenses that sit unused for long periods, especially if they're in closed compartments and/or exposed to long-term heat.

 

I think that's what you're seeing. It's happened to one of my own Leica lenses, two Olympus lenses and a Rollei Schneider lens, all just with the passage of time.

 

The big drawback is that you can't know if it's evaporated lubricants or separation without disassembling the lens and cleaning the inner surfaces; I wouldn't pay any significant amount of money for a lens with that problem if I didn't know for sure what the problem was.

 

If it's just deposited lubricants it'll have no effect on image quality at all; I'd be more concerned about the shutter blades getting sticky.

[bengt_rehn]

Thank you very very much for your responses, Barry and John! I really didn´t know what to do, without your help. The lens will be sent to Mamiya, for a careful examination before I will pay anything. John, do you mean that the lens will NOT suffer in performance, if the colored rings depens on evaporised lubricants inside the lens, without removing the residue on the lens surfaces?

[john_hicks___]

> NOT suffer in performance

 

No, it really won't be visibly degraded assuming the deposit isn't very thick and doesn't provide a growth medium for fungus. But it'll catch any dust or fuzz particle that comes along, so if it is lubricants it really should be cleaned.

 

Unless the price of the outfit is way too good to be passed up, I'd consider the problem with the 75 and the solution to make or break the deal. There are plenty of M6 outfits out there.