[Leica] image quality and format -- 35 v. 120

[Paul Roark <proark@silcom.com>]

On Sun, 19 Mar 2000 Erwin Puts wrote, in part:
>>>If we can 
use a film that has a granularity size and resolving power that is 
below the enlargement ratio of the Leica negative, I dare to say that 
there will be no difference in print quality. That would be the case 
with Technical Pan. <<<

If we look just at grain, then a 35 mm Tech Pan -- with an RMS grain rating
of 5 -- would seem to get close to a 120/MF frame of Tmax 100 -- which has
an RMS grain rating of 8.  

However, depending on how one defines sharpness, the advantage of TP over
Tmax 100 might be much less than Kodak's grain (or resolution) claims would
indicate.  If we look at the MTFs of the films published by Kodak, the Tmax
100 MTF is actually better than that of TP up to at least 100 lines pairs
per millimeter.  For example, at that frequency, the relative contrast of
Tmax 100 is just over 60%, while the relative contrast of TP (developed in
Technidol) is about 50%.  In short, the Tmax 100 might actually have a
perceived sharpness advantage over Tech Pan.  

In my last attempt to move my landscape photography to 35 mm from MF, I
thought that the computer, with its ability to selectively sharpen the
image, would allow me to offset the MTF disadvantage of the 35 image -- a
disadvantage caused mostly by the film size differences.  Additionally,I
decided to move only my wide angle shots to 35 mm.  If you compare the
Leica 28's MTF to the Zeiss 50 Distagon's, you'll see that the Leica at the
edges may win even with the greater magnification required.  (MF SLR wide
angles' off-axis performances are the format's weakest area, optically.)

However, somewhat contrary to my tests and assumptions, in actual practice
the 35 mm shots did not turn out to be as sharp as the MF shots.  I think
the difference relates to the margin of error that MF has, combined with
the fact that in actual practice, as opposed to lab testing, depth of field
is usually important.  Let me explain.

I think that most would agree that the MF records more detail than 35 mm
due to the film size advantage.  However, the issue is really whether a
viewer can see that difference at the enlargement level that one is
targeting.  At 16 by 20, my comparison test prints looked very close,
although some saw the difference in the fine detail.  The extra detail that
the MF negative had was simply smaller, even at a 16 by 20 enlargement,
than the eye could detect.

However, in real shooting, depth of field requirements of three dimensional
landscapes made the negatives of both formats less than perfectly sharp --
unlike the test negatives I'd used in my test comparisons.  In the MF
shots, however, the reduction of the detail caused by less than perfect
focus did not appear to have as much of an impact on the apparent sharpness
of the final image as did the loss of detail in the 35 mm images.  Perhaps
the MF negatives had enough head room to be substantially less than perfect
and still look perfectly sharp at 16 by 20.  (Perhaps this is one reason
the film flatness issue that is so apparent in film testing is ignored by
many, if not most, MF shooters.)  The 35 mm negative, on the other hand,
may not have had that head room -- thus the loss of detail was more visible.

Just my hypothesis, for what it's worth.

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