| Author |
Message |
    
Harryrag
Tinkerer
Username: Harryrag
Post Number: 79
Registered: 05-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, June 13, 2009 - 02:52 am: | 
|
While dismantling a Canon SE 1.7/45 showing signs of lens separation I discovered a thin rainbow-like layer on the last lens inside the rearmost two-element lens group.
Does anybody know anything about this rather strange phenomenon?
What causes this "coating" effect?
Does anybody know whether fungus leaves such colourful traces of its presence? |
Glenn
Tinkerer
Username: Glenn
Post Number: 608
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, June 13, 2009 - 07:27 am: |
|
The phenomenon of Newton's rings, is an interference pattern caused by the reflection of light between a spherical surface and an adjacent flat surface. When viewed with monochromatic light it appears as a series of concentric, alternating light and dark rings centered at the point of contact between the two surfaces. When viewed with white light, it forms a concentric ring pattern of rainbow colours because the different wavelengths of light interfere at different thicknesses of the air layer between the surfaces. The light rings are caused by constructive interference between the light rays reflected from both surfaces, while the dark rings are caused by destructive interference.
The presence of the rings suggests that the rear, two element group, is air spaced. Beyond this I have no other thoughts, only to add that fungus does not produce this effect - well not in the many cases I have seen over the years. |
Harryrag
Tinkerer
Username: Harryrag
Post Number: 80
Registered: 05-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, June 13, 2009 - 09:14 am: |
|
I have known this since my early days of photography when I used to put slides between glass frames, and that is exactly what you describe. Air between the two glasses, of course, but the two elemets were still glued together and the colourful pattern did neither vanish after disassembly of the two halves nor could it be wiped off on the rearmost lens where there was no glue anymore. All other glass surfaces are pristine. |
Harryrag
Tinkerer
Username: Harryrag
Post Number: 81
Registered: 05-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, June 13, 2009 - 09:17 am: |
|
PS: The pattern looks like what you see on those big bubbles when using dishwashing liquid, but on the lens it is permanent. |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 912
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, June 13, 2009 - 02:09 pm: |
|
The pattern is caused simply by the distance between the 2 layers of glass where the cement has separated. The reason it kind of resembles the effect of coating is because coating works in exactly the same way, using the thickness of the coating to establish the distance between 2 reflecting surfaces. Where this thickness is about 1/4 wavelength of light, reflections are suppressed, where it is about 1/2 wavelength they are amplified, and as Glenn explained, a varying distance of separation in this very close range creates a pattern of alternating light and dark bands. Rainbow colors appear because different colors have different wavelengths and so are affected this way at different distances of separation.
If you have this effect on the OUTER surface of an element, rather than in the space between 2 of them, it has to be a foreign substance coated onto the lens either intentionally or otherwise.
I have a simplified discussion of lens coating on my website: it applies equally here if you substitute air space for the coating thickness, and a body of glass where I show the air in front of the lens: http://rick_oleson.tripod.com/index-166.html |
Tom_cheshire
Tinkerer
Username: Tom_cheshire
Post Number: 48
Registered: 04-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, June 13, 2009 - 02:55 pm: |
|
Yikes, dudes, has no one ever see a lens that has been dropped? |
Harryrag
Tinkerer
Username: Harryrag
Post Number: 82
Registered: 05-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, June 14, 2009 - 03:28 am: |
|
Thank you everybody for their inputs on this subject.
I hope quoting from Rick's post helps clarify the matter:
"If you have this effect on the OUTER surface of an element, rather than in the space between 2 of them, it has to be a foreign substance coated onto the lens either intentionally or otherwise."
In this case the INNER surface of the last element is not any different from the OUTER, as the "effect" just happens to occur where separation between the 2 glasses had begun, that is where there was difinitely no glue on the one element but only glass. After taking the elements apart the colourful layer or whatever name can be used here, turned out to be permanent and could NOT be removed.
So the question in my first post is still open, and I hope you do not mind me asking again: what chemical substance or physical phenomenon can cause the effect? The reason for my huge curiosity is that this is not my first encounter with this strange thing and I keep thinking that sombody perhaps can help me understand that. |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 913
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, June 14, 2009 - 08:20 am: |
|
Sorry Harryrag, I misunderstood... your heading referred to Newton Rings between lens elements, so that's what I thought the question was about. Newton Rings do not occur on a single outer surface.
The cause of the rainbow effect on the outer surface would most likely be a very thin (probably 1/10 to 1 micron) layer of material on the surface of the glass. The most likely material under the circumstances would seem to be optical cement (this was a surface that had been cemented and separated, correct?)
Tom, I haven't got the slightest idea what this means: "Yikes, dudes, has no one ever see a lens that has been dropped?"
rick : ) = |
Harryrag
Tinkerer
Username: Harryrag
Post Number: 83
Registered: 05-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, June 14, 2009 - 09:00 am: |
|
Yes, Rick that is right. It was not possible in any way to get rid of whatever that layer consisted of. I once ran across the same phenomenon on regluing a Yashinon on a minister D, but then it was very easy to remove it. This time I had to give up early as I did not want to ruin the lens by polishing. I am a bit puzzled because the glue, or cement, if you like, could easily be removed with acetone. That layer did not even vanish after a night in an acetone bath. |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 914
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, June 14, 2009 - 04:16 pm: |
|
There are different kinds of cement - some are easily removed, some less so, some virtually impossible to remove. |
Gez
Tinkerer
Username: Gez
Post Number: 101
Registered: 09-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, June 15, 2009 - 03:18 am: |
|
If the original cement is a methacrylate compund, soaking for a couple of days in dichloromethane solvent will shift the residue. |
Casual_collector
Tinkerer
Username: Casual_collector
Post Number: 8
Registered: 04-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, June 15, 2009 - 03:19 pm: |
|
I dropped a lens once. The impact separated the front doublet causing the interference ring effect. I looked at the lens some months later and the rings were gone. I assume some oil from the focus helicoid filled the gap caused by the fall. |
