[rollei_list] Re: 120 Film flatness, Rollei TLR plane glass, etc ..


----- Original Message ----- From: "CarlosMFreaza" <cmfreaza@xxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Friday, August 19, 2011 7:07 AM
Subject: [rollei_list] Re: 120 Film flatness, Rollei TLR plane glass, etc ..


I think your diagram works perfect for 220 film, there is a physical contact with 120 film between the film paper back and the "pressure plate" most of the times, I did a test some years ago putting a fine cream layer on the plate and the film paper back touched the cream along the film travel, it does not mean the plate "presses" the film BTW, but it is showing the paper could be a problem for the film flatness. In fact, there is a very perceptible difference when you advance the film with 120 film and with 220, the film advance mechanism can be pretty heavy with 120 film sometimes, it is caused by the film paper back resistance to advance brushing the pressure plate and the paper additional weight, in the other hand 220 film passes "clean" along the film channel and you tend to believe there is no film in the camera, the film advance mechanism becomes very light, I used 220 film with the 3.5F, 2.8C and Rolleicord IV and the difference
is similar for the three cameras.

Chris Perez lens resolution tests for the Rolleiflex TLR are not convincing to me, he did a good job but the numbers show differences too large amid some lenses and unbalanced performances too much for the same lens I never saw in my photographs, he used the regular viewfinder for focusing and I suspect some issues with the parallelism between the lens board and the focal plane for the cameras used, as he perceived for the Schneider Xenar test, my 'cord IV Xenar has an excellent sharpness at f4 as you can see in some of my portraits taken camera hand-held. For example, the Xenar test shows the best performance at f 22 with identical figures for the three measurements, this is impossible for a camera-lens combo with right adjustment, the Xenotar 2.8/80 shows some impressive numbers, however at f5.6 they are 120 68 17 showing an abnormal difference from the center to the edge for a standard lens, f5.6 is one ofmy preferred f stops with the Xenotar and I never saw a so big difference in my
photographs for the center and the edges.-

Carlos

There are a lot of issues with testing lens performance on a camera because one is really testing the performance of that particular camera as a system. The primary problems are film flatness and the behavior of the focus system. It is quite possible for the finder and taking lenses not to match plus its quite possible in many cameras for one or both to be tilted. Another factor often overlooked is that the variation in optimum focus due to variation in spherical aberration (focus shift) of the taking lens means that the two match only at the stop at which the finder was adjusted. Actually, this applies to any camera where focus is determined by some means other than observation of the actual image from the taking lens _at its intended stop_. For the most part I think Chris Perez's tests are valid and are quite interesting but their limits and possible errors must be understood. I think he does understand them but some reading the results without looking a the details of the tests may not. Another problem, which I think most people understand, is that a resolution test on film measures a combination of the lens resolution and film resolution. While a couple of simple formulas for the combination can be found in fact the two do not combine in any simple manner; it s a convolution of the two although usually something like the square root of the sum of the squares comes close enough. I will add to this that the resolution charts given by some manufacturers seem arranged more to obscure lens performance than elucidate it.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk@xxxxxxxxxxxxx
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