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Jani Heikkinen
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, August 14, 2004 - 08:24 am: | 
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Hi!
When looking at a goerz celor lens I noticed a yellow coloured reflection and some rainbow coloring on the rear lens. First I thought it is probably separation, but celor doesn't have any cemented pairs. Are these kind of reflections normal for a celor lens? |
Winfried
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, August 14, 2004 - 02:53 pm: |
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I don't know this particular lens. But the glass alloys used on some older lenses sometimes to corrode a bit on their surface, and the colors you see probably are caused by light interference of thin layers.
I once noticed a blueish shine on a lens from the 1930s that never has seen a coating plant, but it was confirmed by others that some lenses get a layer of corrosives looking like a coating. In an old book about optics it was mentioned that some glass alloys are corroded on purpose to achieve an anti-reflective surface. |
Jani Heikkinen
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, August 14, 2004 - 09:56 pm: |
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I think that could be it. It looks like yellow coating, but as the lens was made in 1898 I doubt it was one of those modern coatings. But still some of the rainbow reflections are puzzling. And these too only show when looking at the lens at a specific angle. |
Winfried
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, August 15, 2004 - 02:35 am: |
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This is typical for interference of light at thin layers. I think you already checked this but maybe there is a very thin layer of oil or grease on the lens? |
Jani
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, August 15, 2004 - 04:14 am: |
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I cleaned the surfaces with opticlean and there shouldn't be any traces left. But could it be possible that it isn't actually on the surface it seems to be. |
Winfried
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, August 15, 2004 - 10:04 am: |
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Yes, sometimes it is hard to see on which surface dirt etc. actually is. Usually you can check this by touching the surface with the tip of a toothpick and check whether the tip reaches the things that seem to be on that surface.
However, as long as the lens looks clear when looking through it this stuff on the surface hardly won't affect image quality. I just made some shots with a slightly foggy lens (a rare wide-angle lens for a french Foca rangefinder), and it performed quite well for the USD 2 I paid for this grab-box item... |
Charles Fallis
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, August 20, 2004 - 06:43 am: |
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Actually, I've been reading that the lens coatings today are synthetic versions of the natural coatings that used to develop on old lenses. Back then the old lenses that had developed this coating (called a "bloom") were highly valued. You might want to reconsider removing it. |
Jani Heikkinen
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, August 20, 2004 - 06:46 am: |
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Thanks for help. I'll just let the lens be as it is. |
Winfried
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, August 20, 2004 - 01:55 pm: |
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Charles, it is more or less true what you have read. It is correct that some photographers noticed very early that their images were more contrasty when the lenses had a slight layer of corrosion or other foreign stuff causing less reflection. The english Taylor company received a patent on such coatings in the 1910s or 20s although they did not know how to manufacture them artificially.
Zeiss (and Kodak) made a scientific approach to that problem in the late 1930s and found that a layer of certain thickness of magnesium-fluoride (MgF) has all the properties the old photographers desired, and also developed a process how to apply such coatings. |
