[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

2011/8/19 Emmanuel Bigler <Emmanuel.Bigler@xxxxxxxx>:
>
> hello all !
>
> I take the liberty to change the subject to the film flatness issue
> and the (in)famous Rollei TLR plane glass.
>
> I made some time ago a rough diagram to explain how I see the 120 film
> positioning in the Rollei TLR, with and without the glass plane.
>
> http://farm4.static.flickr.com/3411/4573709647_e2452f3150_z.jpg
>
> The specs say that in the standard, basic configuration, the film is
> allowed 0.45 mm of "freedom" between the "(non-)pressure" plate and
> the inner rails. Note how the glass plate should be positioned with
> 2/3 inside and 1/3 outside the inner rails to compensate for
> refraction effects.
>
> I think, in addition (or I should I say : addiction !! Isn't the
> Rollei-TLR-mania an addiction ? ;-);-)) to what has been said, that
> the F-number in use plays a major role in final image sharpness,
> whatever any detrimental "film bulging" issues might be.
>
> My understanding is that the Zeiss study related film flatness for
> the Contax 645 was triggered by the use of f/2 lenses for which the
> depth of focus at f/2 is 4 times more critical than for a TLR stopped
> down to f/8. If we take a conventional depth-of-focus model, with a
> very stringent circle of least confusion of 30 microns like for 35 mm
> photography, we end up at f/8 with a depth of focus of plus or minus
> 8x30 = plus or minus 240 microns, by a strange coincidence 2x240 = 480
> very close to the 450 microns in the gap between the "pressure" plate
> and the inner film rails.
>
> Chris Perez in his famous tests
> http://www.hevanet.com/cperez/MF_testing.html
>
> where he reaches over 90 cy/mm at f/5.6 and f/8 with a well-tuned Rollei TLR
> planar or xenotar, does not use the plane glass (I think I got this
> information from him by private e-mail).
>
> But we do not know how he winds his film up during tests ... since it
> was a series of shots, probably the film has no time to "relax"
> between shots ....
>
> --
> Emmanuel
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