Lens coatings

[Saulius]

I am looking to purchase a used lens,a 14'' F6.3 Commercial Ektar lens. My quest ion regarding this and other used lenses is when describing it the sellers says something like "Glass is good with a few minor cleaning marks". Firstly, when i nspecting a lens what should I look for, how can I tell if the coating has been "damaged", rubbed off to some degree. Second, at what point is there too much d egradation that the coating is useless? At this point should the old coating ju st be polished off? Also, do cleaning marks effect the overall image aside from less reduction in flare? I appreciate any comments on this topic.

[Shigehiro Ishii]

This is just my personal opinion. Slight cleaning marks will not affect the overall image. They say the coating has nothing to do with shrapness but just preventing flare. Lens fog will affects image. Anyway any old lenes make less contrast than modern MC lenses. The production year of EK's Commercial Ektar is related to the word, "CAMEROSITY" which is equivalent to 1234567890. If the lens stamped as RC 607. That means it was made in 1951.

[Martin F. Melhus]

I'd like to offer a slight correction to the comments of Shigehiro Ishii (shige-i@tkd.att.ne.jp). He states that the coatings on lenses have nothing to do with sharpness. This is incorrect. Coated and multicoated lenses are treated to correct chromatic aberration. CA is a lens error in which light of different wavelengths (or colors) is focussed to different distances. This is a standard problem with simple glass lenses. Around WWII, techniques were devised for coating lenses so that the colors all focussed to the same diatance. See Jim Stone's book "A User's Guide to the View Camera," pp. 115-116, for a more complete explanation of chromatic aberration (both axial and transverse.)

So an uncoated lens will have more difficulty focussing any image with different colors of light in it. Colored fringes will appear on color film; with B&W film, there will just be general unsharpness.

Aside from this quibble, I agree with Shigehiro Ishii's comments, and appreciate the info about the serial number codes.

Regards,