Quite some time ago Ctein wrote that he had discovered
that visual focusing of enlarging lenses resulted in blur
due to chromatic aberration. He blamed it on the UV part of
the spectrum. I was one of those who poo-poo-ed the idea.
Maybe not. I found the following in a Kodak booklet entitled
_Use Maintenance, and Repair of Professional Equipment_
which is included in the _Kodak Professional Handbook_
dated 1952. The particular section is on the maintenance of
the Kodak Auto-Focus Enlarger, Model E. This is a 5x7
enlarger which looks something like the Elwood but uses a
different method of auto focusing.
"These enlargers have been shipped with two types of
lenses, the Kodak Anastigmat No.33 f/4.5--7.5 inch, and the
Eastman Projection Anastigmat f/4.5--7.5 inch. In the case
of elnargers fitted with the No.33 lens, correct focus may
be determined by photographic means, but not visually,
because there is a difference between the photogaphic and
visual focus."
The No.33 K.A. is a Tessar also used on cameras. The
Projection Anastigmat was later called an Enlarging Ektanon,
the change in names being made around 1947. The No.33 Kodak
Anastigmat is a very old design, probably dating from the
mid-1930's, maybe earlier. I suspect that most high quality
enlarging lenses made since perhaps the mid 1950's have much
better chromatic correction than this lens, but perhaps not
all of them. Since the booklet covers enlarger models which
remained in production for several decades the lenses
mentioned are older than the booklet
Many modern lenses do not have much transmission in the
UV either because the glass itself absorbs it or because the
cement used for cemented elements absorbs it, or both. I am
still very skeptical of Ctein's findings, especially since I
have heard from no one else who has been able to duplicate
it, but I am, perhaps, a bit less skeptical now. This is
perhaps also a good reason to use a UV filter in the light
path of enlargers, especially those using cold-light heads,
which can have significant output in this region.
While I am quite willing to use old lenses on medium or
large format cameras I prefer reasonably modern (as modern
as I can afford) lenses for enlarging of all formats.
--- Richard Knoppow Los Angeles, CA, USA dickburk@xxxxxxxxxxxxx
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