Archived posting to the Leica Users Group, 1997/10/07

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Subject: OFF-TOPIC; Blue filter enlarging stuff
From: Joe Berenbaum <joe-b@dircon.co.uk>
Date: Tue, 7 Oct 1997 18:36:16 +0100 (BST)

>>Gene Nocon--a gentleman who runs lab across the street from studio and
>>has printed for many greats uses a blue filter in his focusing tool     
>>Without it he says prints arent truly focused  He showed me print
>>coparisons and he is right    Probably big factor at wider F stops  
>>Apparently this is standard knowleddge among high level lab types   When
>>I get back into do my own printing I will definitely get the
>>filter       maybe APO lenses dont need it   I`ll ask when I return
>>Donal Philby

I've just got the book by Gene Nocon "Photographic printing" and in it he says;

"Using the blue filter on the Omega Micro-grain focuser; at this enlargement
the grain can be seen to be pin sharp on the print. The image projected onto
the easel will actually appear out of focus after focusing with the blue
filter, but the print will be sharply focused.
Without the blue filter on the grain focuser the image appeared to be in
focus. The resulting print, however, is out of focus."

This suggests to me that it is not a question of using a blue filter to
increase contrast or reduce brightness, but that the image that appears on
the paper when printed is formed from predominantly blue light or light that
is focusing in the same plane as blue light. Otherwise, the image in the
above description that was focused without the blue filter would be in focus
when developed! 

What is not clear is how this applies to VC paper. The answer, or some of
it, may be in Ctein's Darkroom User article (Darkrrom User, 1996, #1) on the
best enlarging lenses, where he says;

"I recently discovered that there can be huge differences between the plane
of best focus seen by our eyes and the proper focal plane for VC print
paper. I've seen as much as a 15mm focusing error when printing 10x8's from
my 35mm negatives, using my Beseler Dichro 45 color head, Computar 55mm f1.9
lens and Kodak Polymax paper. That shirt produces a print which is visibly
unsharp! So far most other printers have not found a serious focus problem,
but "most" is far from "all"!
The cause is longitudinal chromatic aberration. Longitudinal chromatic
aberration causes different colrs to be focused at different distances from
the lens. As an image is moved in and out of best focus, the final detail
will (for example) look magenta in hue on one side of the best focus
distance and and green in hue on the other. Good enlarging lenses are
adequately corrected over the red-green-blue part of the spectrum.
Unfortunately modern VC papers respond well to much shorter wavelenghts of
light, down into the near UV where the human eye sees almost nothing. As a
result, even when we focus our enlargers through a deep blue filter, we're
using a diffeent part of the spectrum than the print paper sees. The print
paperresponds to light outside the normal spectral range for which the lens
is optically corrected. 
Not all enlargers put out enough violet-UV to cause problems. Not all VC
papers are sensitised this extremely. The quality of LCA correction for V-UV
light varies greatly with the lens design. Consequently you may not have any
problem, but that's far from a certainty....
...I'll be reporting on this problem in much more detail in a few months
time in this magazine. Meanwhile I suggest you run a careful focus check on
any lens you're considering buying, if you print with VC papers. I'd look
especially carefully at lenseswhich I report as having larger-than-average
amounts of longitudinal chromatic aberration in the visible spectrum."

Interestingly, in this article's top ten lenses test results, the Rodenstock
Apo-Rodagon N 50mm/f2.8 has "worse than average red-green longitudinal
colour." Fascinating...

Joe Berenbaum