lens spots

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i have a lens question. can someone tell me what an oxidation spot on a lens element is/looks like? thank you. or, why would a 12" Goerz lens with uncoated elements, which are very clear, have an almost un-detectable appearance, to the otherwise clear front element, of gasoline spilled on water. thank you. this isn't the rare-earth radio active type of glass one reads about, is it? thank you.

-- david clark (doc@ellensburg.com), June 18, 1999


Hi David, I will try to answer to your question(s), starting from the last one. I don't think your Goerz lens is radioactive, because from what you say it shold be quite old and at the time of the construction the use of rare-earth oxides for the optical glass was almost unknown. In any case, if you want to be sure, place your lens on a sheet of lith film (front lens down on the emulsion side), live it on the dark for 24 hours and develop the film. If you can notice the round shape of the lens on the film as a dark spot, there is radioactivity, otherwise no radiation (of course, if you see the lens image as a clear spot, this means that you have light leaking in your darkroom!). The optical glass "oxidation" (which chemically speaking in effect is not a real "oxidation") is the result of very slow hydrolysis ractions between the silicates of the glass and the moisture of the air; the rate of the process depends from the environmental conditions (humidity, temperature, exposure to acid or basic media) but the result is that the silicates transform in oxides, and the surface layer of the lens changes his refraction index. At the beginning the oxide layer is very thin, and makes a non regular interference fringes pattern (exactly what you say like gasoline spilled on water); after many many years the oxide layer becames more thik and even, but the appearence changes to a general withish-gray fog (by the way, you could notice interference fringes also on the inner part of doublets, but they are related to a partial detachment of the glue between the two lenses, and normally they have a "shell" appearence). For the surface oxidation layers, the best way to remove them is to make a mechanical repolishing with special machinery, keeping the original curvature radius of the lens and using finer and finer grindig pastes. The treatment is rather long and costly, it must be done by specialized people. If ther layers are still rather thin (i.e. they show colors) normally they don't do any arm to the final image; a risky operation could be done at home, using at room temperature a dilute solution of nitric acid (some time a diluted solution of hydrochloric acid works better), dipping for 20 seconds, rinsing, drying , checking, dipping again etc. But the final effect of the operation can span from a good succes to a complete disaster depending from the strenght of the acid solutions and the composition of the glass. You could try it only if your are mentally prepared to the possibility of transform your almost-sharp lens to a very soft-focus lens, good for portraits without wrinkles to elephant faces! Have a nice summer, Franco

-- Franco Rallo (f.rallo@ele.uniroma3.it), June 18, 1999.

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