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Different colors Contax MC UV filters green vs blue



I noticed my multi coated UV filters for my Contax G lenses have different colors. My newest have a deep green coating and some others (some are bought second hand) have a more blue color! What does this mean? Are the ones with the more blue coating deteriorated? Do I have to replace them with new filters? How bad is this? Is it still better then single coated filters?

I don't want to work with bad filters, but I don't want to replace filters that are actually still in good shape, so your input is appreciated!



The "color of the filter" depends on the type of coating and in particular the coating thickness and the angle at which you view the glass (and therefore see the color). The color you observe is caused by interference of the light reflected from the various interfaces of the glass and coating material layers (so the front of the glass, the backside of the coating and the backside of the glass). Please note that only a 5%-10% difference in thickness of the coatings on the different filters causes a very different color. I have professionally been active in the business of producing these ultra thin films on silicon, glass and many more materials and I have a lot of experience with the "thin film" color effect. It is not possible to conclude about deterioration of the layer or filter from the coloration only. Deterioration usually manifests itself in dull scratched surfaces. As long as your filter is free of scratches and nicely "shines" under glancing view I cannot see why you should not use it.
There is one exception to the above: if there is a very thin layer of contamination on the surface of the filter (and a layer of "fat type dirt" of only 10-20 Angstrom is really enough) there will also be a distinct color effect, but usually this will also render the filter surface dull and less shiny. Cleaning is the only option to check this. A clean good brand UV filter has over 99% transmission and very low reflectivity of light not parallel to the optical axis as well as no coloring of the pictures made and therefore the quality for photographic purposes is the same for the filters with the various types and thickness of coatings as long as you use good quality brands: Minolta, Zeiss, Leica, Contax and B&W (not in this order) are the "good" guys here. If you chose these brands and they look good without scratches you are in good shape.