Re: STARTING POINT
Speaking from personal experience and being a convert to Leitz enlargers
some years ago, I will note that the difference between focusing with my
eye and setting up an enlarger which is "pre-focucused" using test
prints (the focus of the enlarger being "locked in" based on test prints
examined through a loupe) is quite astonishing. I don't know what the
weak link is, but the difference in sharpness between prints made using
an on easal "grain magnifier" (with or without a blue filter) and an
enlarger with "pre-calibrated focusing" is quite significant the later
being the clear winner.
--greg schmitz
Don Bryant wrote:
Don,
Ctein reports in Post Exposure that some vc
emulsions are extraordinarily sensitive to the uv end of the spectrum which
the human eye can't see to focus.
As I recall Ctein was probably the only person that could detect the
focusing shifts he reported.
Or to put it another way I could never discover or replicate the focus shift
in my darkroom.
Patrick Gainer posted the follow message on Photo.net, Sept. 19, 2001;
'There was an article in Photo Techniques some time ago (Jan/Feb 1997)called
"Hazards of the Grain Focuser" written by some crazy guy named Pat Gainer.
He proved by a series of tests that it is best when using a focusing device
where an aerial image is made to coincide with a crosshair to use white or
green light for focusing. The chromatic aberration of the eye is the
culprit. Dig up the article for a complete description of the experiments.
Using a deep blue filter for focusing resulted in an average error of about
9 mm in the direction reported by Ctein.'
So Ctein's conclusion has been suspect for various reasons. A simple test
with some RC paper should reveal the truth about the presumed UV focus
shift.
Some individuals that contact print large format negatives report that
contact prints made on graded paper are sharper than contact prints made on
RC paper. Again I've never seen the difference.
I think Edmund Scientific at one time sold a film test targets that could be
used to contact print onto film to test the resolving capability of a
particular film. Perhaps the same approach could be used to compare the
relative sharpness of VC papers and graded papers.
Recently Jim Galli posted a comparison of different antique lenses using
Kodak Polycontrast paper on his web site. Take a look at his shots, he never
mentions focus shifts but perhaps he wasn't being critical enough.
http://tonopahpictures.0catch.com/Apricot_Blossums/Apricot_Blossums.html
Don Bryant
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