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

  • From: Emmanuel Bigler <Emmanuel.Bigler@xxxxxxxx>
  • To: rollei_list@xxxxxxxxxxxxx
  • Date: Fri, 19 Aug 2011 12:12:18 +0200


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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