[rollei_list] Re: Veiling Flare


----- Original Message ----- From: "John Owlett" <owl@xxxxxxxxxxxxxxxx>
To: "Rollei List" <rollei_list@xxxxxxxxxxxxx>
Sent: Friday, June 24, 2005 9:33 AM
Subject: [rollei_list] Veiling Flare


Thank you Les and thank you Emmanuel for your welcoming me to the
RUG.

During the decades when a Rolleiflex Standard 622 was my main camera,
I was always delighted with the way it would produce with ease a size
of print that would be a struggle for 35mm. On the other hand, when
shooting contre jour, which I like to do, with the uncoated Carl
Zeiss Jena Tessar, there really was quite a lot of veiling flare
in the negatives.

So I went over to the dark side and bought a Nikon. Using a Rollei
had given me buildqualititis and most 35mm SLRs seemed to me to be
made of sandwich boxes, but the F3/T was acceptably robust. 1980s
Nikkor lenses with "Nikon Integrated Coating" (which used, so I've
been told, coating technology licensed from Carl Zeiss and from
Asahi Optical) tamed the flare.

When I got the test negatives back from my "new" 3.5F Whiteface, my
chortle of delight came to an end. On one frame the shutter had
failed to open; the taking lens had consistently focused closer than
the viewing lens; there was some fogging of the spaces between the
frames; and the frame spacing was erratic. Only teething problems,
to be sure, and someone less demanding might have asked for them to
be repaired under warranty, but there were too many teething problems
for me so I took the camera back for a refund.

The reason for this saga is that the contre-jour negatives also
showed a certain amount of veiling flare. Nothing like the amount
I would get from the uncoated 1930s Tessar, but more than I'd expect
from multicoated 1980s Nikkors. When I resume my search for a 3.5F
Whiteface, and I shall, am I the sort of person who would be better
served by the multicoated Planar rather than the single-coated
Xenotar? Or am I being confused by the teething problems with a
particular camera?

Later,

Dr Owl

----------------------------
John Owlett, Southampton, UK


Veiling can come from other sources than lens flare. The main one is reflection of extra-image light from the inside surfaces of the camera. In the early 1950's Rollei began to use internal baffles to reduce this effect. The are effective but can cause some reflection themselves. Extra-image light can be reduced by using a lens shade. Rollei shades are good because they are designed for the lens and format so cut off pretty close to the limit of the image.
Lens flare takes two forms: an overall veiling; ghost images. The veiling comes from all light getting into the lens so a lens shade will have only a small effect on it; most comes from image light bouncing around in the lens. Ghost images are caused by unfortunate combinations of reflecting surfaces in the lens and are mostly a lens design problem. Coating supresses them but may not get rid of them. The Zeiss Tessar used on Rolleiflex's seems not to suffer from this problem. Actually the old f/4.5 Tessars that were used on Speed Graphics also don't have ghost images. I think probably this was something the designers were aware of. OTOH, early, uncoated Kodak Ektar 127mm, f/4.7 lenses, as used on the same cameras, do have a minor ghost image of bright objects near the center of the image. It does not appear in the coated version of the lens.
Lens coating has a noticable but small effect on Tessars because they have only six glass-air surfaces. Because flare increases nearly geometrically with the number of surfaces its quite noticable at 8 glass-air surfaces, so coating makes a noticable difference for Planar/Biotar type lenses, and also for the five element Planar types like the Xenotar.
Single coating works best at one wavelength of light. By using multiple coats the bandwidth of effectiveness is broadened out. Also multiple coating seems to suppress reflection more than single coatings even at their most effective wavelength. The effect for relatively simple lenses is probably more noticable for color work whre the absense of flare improves color purity and saturation as well as overall contrast. In more complex lenses the need for coating is imperative. Modern zoom lenses would not be possible without effective coating.
Anytime I see excessive veiling I check the lens for internal haze. Many lenses develop this haze even when not too old. I don't know the source for certain but it may come from stuff evaporating from the anti-reflection paint inside the cell. Schneider lenses seem more suceptible to it than Zeiss but most lenses can have it. Shine a flashlight through the lens to see the haze. even a small amount of haze will destroy the image contrast. The haze is easily removable with ordinary lens cleaner but it may take some work to disassemble the lens cells to get at the surfaces.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx

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