Re: [Leica] 35mm versus 120

[Paul Roark <proark@silcom.com>]

At 12:01 AM 3/19/2000 -0800, Mike Quinn wrote:
>...
>But what do experienced MF shooters do to keep their film flat?
>
>Paul Roark wrote:
>
>> Experienced medium format shooters learn how to keep the film flat.  35
>> film will also not lie flat where it has been pinched by the film can too
>> long.

The model I use that seems to explain and predict film flatness well,
according to my resolution tests and experience shooting Rolleis and others
for 20 years, is that is that the film "remembers" the curves it goes
through prior to getting to the film plane.  The tighter the curve and the
longer it has sat there, the more it will remember it.  Likewise, the
longer it sits on the film plane, the more likely it will "forget" the past
curves -- some say "relax."   Film thickness, type, and even humidity also
affect the problem.  

For evenness on a medium format (MF) camera, it appears that the uniformity
of the film's tendency to curl is a critical factor.  If the film has been
sitting for a while and is then wound to the next frame and shot
immediately, it will have part of the frame "remembering" the curl of the
roller.  That will throw the frame off the film plane by differing amounts
depending on lots of factors, including the extent of the past curl.  The
worst I ever saw in real world shooting was when I  shot an English
cathedral steeple with my Rollei SL66 on a very cold morning.  The frame
that had sat on the reverse curl put an out-of-focus line across the frame
(and steeple) that was visible without a loupe.

When I'm testing a lens -- usually  with Tmax 100 or Agfapan 25 (but this
works with most films and MF cameras) -- and I want to be sure that the
film is flat, I simply waste the frame that has been on the roller.  So, I
shoot a blank, immediately go to the next frame, wait 2 minutes for the
film to "relax"/"forget" its momentary stay on the roller, and shoot.  I
don't think I've ever seen this procedure fail with the films I use -- with
one exception.  Tech Pan in high humidity is so hard to control that I've
abandoned it for MF shooting.  The emulsion appears to swell, and the film
base has so little strength to offset this, that it will randomly come off
the film plane with every procedure I've used -- the "reverse curl" back of
the SL66 being worse than the TLR or rangefinder type cameras I've used.

In the real world, when I'm shooting MF I simple keep track of how long the
film has sat on the roller.  When shooting a series of shots fairly
quickly, the issue can be almost ignored.  When I do landscapes, want
maximum sharpness, and the film generally sits still for some time while I
hike to a different location, I end up shooting every other frame.

One person on this thread asked about the Fuji GA645.  I have one, and it
has the least problems with film flatness that I've experienced with an MF
camera.  (The older GS645 was not as good.)  On the GA645 Zi Fuji put two
rollers on the face of the film just outside the frame that hold the film
on the pressure plate.  It appears to work well.  I've found I can shoot it
like a 35, virtually ignoring the film flatness problems that are usually
associated with MF shooting.  (Moreover, that Fuji zoom is, in some ways,
better than my Zeiss glass -- amazing.)

Paul Roark
http://www.silcom.com/~proark/photos.html