[pure-silver] Re: Uncoated lenses
----- Original Message ----- From: mmagid3005@xxxxxxx 
To: pure-silver@xxxxxxxxxxxxx
Sent: Tuesday, December 30, 2008 8:26 AM
Subject: [pure-silver] Uncoated lenses
I want to use an old f/6.3 uncoated Tessar from a 9 X 14 ICA folding camera. Is there a filter I can use that will simulate the effect of a coated lens? 


Marty
There isn't. If any sort of simple filter could accomplish this coating would not be necessary. Lens coating is made necessary by the "mis-match" between the index of refraction of air and that of glass. This causes some energy to be reflected by the surfaces. The coating is such as to obtain a better "match" and reduced the amount of reflection. The reflections take two forms: one is an overall flare, that is, light from the image is reflected many times by the surfaces within the lens causing an overall fog, thus reducing the contrast of the image, especially in the shadows. When photographing in color there is also a reduction of color purity because reflected light of various colors get spread around in the image. The other effect is "ghost images". These are semi-focused images of bright objects in the image or just outside of it. They are caused by reflections from surfaces with the right curvature to focus the reflected light at the image plane. Some flare is caused by extra-image light, this can be reduced by using a lens shade but most of the flare is caused by the same light that forms the image so a lens shade is ineffective for it. Lenses can also cause flare due to internal reflections from the lens mounting. Coating will not reduce this although a good lens shade can. Such reflections should be largely eliminated by proper baffling of the internal surfaces of the lens mount but this is often not the case in older lenses. The internal surfaces of the camera can also cause flare. The baffling in the post about 1953 Rolleiflex and Rolleicord are an attempt to eliminate this source. The amount of flare from uncoated surfaces increases very approximately geometrically with the number of surfaces. A Tessar or Triplet is about the most complex lens where the flare is reasonable, that is, six glass-air surfaces. In a Planar/Opic/Biotar type, or any "double Gauss" lens with eight glass air surfaces the flare begins to become significant, especially for color. Lenses with many glass-air surfaces, such as modern zoom lenses, would be impossible without good lens coating. Early coating was a single layer of a material with an index of refraction about half way between glass and air and about 1/4 wavelength thick at the color at which it was most effective. The effectiveness falls off at both longer and shorter wavelengths. Since the band of colors of interest in photography is only slightly greater than an octave wide a single coating is relatively effective. However, by stacking coatings of the right materials and right thicknesses, the effective bandwidth can be substantially increased so that modern multiple coatings can almost eliminate reflections over the entire visible color range. The effect is most noticable for color work where both shadow contrast and, especially, purity, is very substantially improved. Another source of flare is haze inside the lens. This affects coated and uncoated lenses. I don't know the source of the haze for certain but I suspect it may come from material which evapoates from the anti-reflection paint inside the cell. For the most part this haze cleans off with ordinary lens cleaner but the difficulty is getting access to the internal surfaces, this varies with the lens mounting arrangement. Even a slight haze will destroy the image contrast. A good way to check for haze (and bad cemented surfaces) is to shine a flashlight through the lens. Any visible haze is too much. Again, no filter will duplicate the effect of a coating alghough uncoated filters will cause some additional flare themselves. In general, Tessars are low flare lenses. Ghose reflections depend on the individual design, for example, Zeiss Tessars do not have any strong ghost images but early, uncoated Kodak Ektars of the Tessar type have a strong ghost image of bright objects in the image field. This is not visible on the later, coated, versions. 

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Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 
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