I just found an interesting post
http://cdegroot.com/archives/yashicacontax-slr/199811/msg00086.html
"Dear colleagues,
"before the discussion about the sens or nonsens of the RTS III's vacuum back
is escalating into a debate about a snobbish attitude of Contax users (and
please bear in mind that this list was originally founded as a Yashica list
- and I have to think hard to find some less snobbish camera owners than
Yashica photographers - maybe Praktika users? Many Contax users have
started by slowly and painfully upgrading an existing Yashica system, either
by buying some - mostly second-hand - Zeiss lenses for their Yashica bodies,
or by adding a Contax camera and still using their Yashica or 3rd party
glass with it, so I think most of us Contax users know very well that
slightly arrogant smile some over-equipped Nikon/Contax/Minolta etc.
photographers bear in their face when they meet someone handling his or her
small FX-3, FX-D, FX107, FR-I or whatever simple and old Yashica body we had
in our bags. I do not think that Contax photographers are more arrogant than
the users of other system brands, but maybe the fact that we have to save
money considerably longer in order to buy a single lens, and that we are
enjoying this purchase more intensely and for a longer time sometimes is
misinterpreted as snobbish attitude. I am convinced that a Canon or Nikon
photographer who has just spend a sum which would be enough to buy a small
car on a 800 or 1200mm lens also will readily talk about its wonderful
quality in the most flowerish words - without being a snob.) I would like to
put down my thoughts and experiences with this vacuum device. Most of it has
been written a long time ago in the rec.photo newsgroup and can be still
read on Steve Scary's Contax website.
"During a meeting in Les Baux in 1979, Dr. Kaemmerer of Carl Zeiss spoke
about the possibilities to improve picture quality in modern large aperture
lenses and referred the results of measurings Zeiss had made on the flatness
of film. Most film materials showed deviations from total flatness up to
80um (Mikrometers) which mostly was found in the center of the picture frame
(therefore it would be totally useless to locate the RTV's openings near the
egde of frame!). This deviation is responsible for a reduction in contrast
of about 60% when measured with an 1.4/50 lens wide open. The reason for
this problem is not the film material itself but the mound of the film
cartridge, which is bending the film material in this problematic manner. If
the film is expended rather quickly, then the bending is not perpetual, but
if the camera is left with the loaded film for some days (as it is quite
often with us hobby photographers), the two or three frames which are bended
by the cartridge mound do not lose their deviation when they are pulled into
the picture frame behind the shutter curtain. If they are exposed with a
wide-aperture lens then the chance that they are not sharp in spite of the
viewfinder showing exact focussing is rather large.
"Quite a time ago there was the thread on the rec.photo where Nikon F 4-users
complained about a possible "design flaw" in their cameras because some had
experienced several cases where the second, third and fourth picture of a
day's shooting session were not sharp. In fact they were not victims of a
camera design flaw but rather of a film cartridge design flaw. In his speech
Dr. Kaemmerer stated that Zeiss had made some tests with a slightly modified
cartridge in order to minimize this bending problem, with good results.
According to him Zeiss had contacted several film producers and asked them
to incorporate these modifications into their film cartridges, but without
result, as we now know. So ten years later the Kyocera engineers brought us
the vacuum back in the RTS III to solve the problem which would not have
been one if the film producers had done their homework.
"I could personally make out the difference when I sometimes realized that
pictures I had made with a 1.4/85mm on my ST were not sharp in spite of me
being absolutely sure of having focussed as exactly as possible. So I
contributed those spoiled pictures to a camera shake or my inability of
focussing that lens if used wide open (f1.4 or f2.0). After I had bought the
RTS III I never experienced these problems, so I did not think about it any
more, but when I read the printing of Dr. Kaemmerer's speech, I retrieved
some of the unmounted film strips I still had and - thanks to the ST's data
back - I realized that indeed the unsharp pictures where numbers two and
three (I am not sure about no. four) of the day, and the camera had not been
used for at least three days.
"So I had to conclude that the RTS III's vacuum back indeed has a positive
effect, but in such few and special instances, that itself the RTV never
would justify the camera's high price. To be honest, I would prefer a more
versatile databack as that available for the AX against the vacuum back, but
since it is built into the camera, it is nice to have it. If the RTV is
deactivated when using lithium cells instead of AA batteries should not
worry anybody - as long as you tend to shoot Pulitzer-prize winner pictures
with an 1.4/50 or 85mm wide open in the beginning of the day!
"Please excuse my lengthy response,
Marcus
Dr. Marcus Hanke
European Legal History Dept.
Faculty of Law
Salzburg University
Churfuerststrasse 1
A-5010 SALZBURG, Austria"