[pure-silver] Re: Uncoated lenses
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Tue, 30 Dec 2008 13:18:55 -0800
----- 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.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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