Richard Knoppow a écrit :
Richard and Ralph
----- Original Message ----- From: "Ralph W. Lambrecht" <info@xxxxxxxxxxxxxxxx>
To: "PureSilverNew" <pure-silver@xxxxxxxxxxxxx>
Sent: Thursday, April 27, 2006 4:34 PM
Subject: [pure-silver] Re: Chromatic Aberration of Enlarging Lenses
I achieve the best focus by focusing with white light only. Then, I swing
the MC filters into the optical path for the exposure. Focusing with Y/M
light creates a focusing problem, due to the chromatic aberration of the
human eye, which gets worse towards red. Focusing with white light seems to
fix that. However, the use of a blue filter turned out to add no benefit to
the white-light focus in any of my tests.
With some people the chromatic aberration of their eyes is so bad that they
cannot focus on red light at all. If you have a red-LED alarm clock, try to
focus on the timing display and the surrounding area at the same time, it
might be difficult to do. This is worse in dim light, because your eye's
aperture is around f/2 in dim light. In bright light, it gets up to f/8 and
you get the benefit of depth of field.
Another example is a newer model-year VW's instrument panel (dash board).
The designers decided to use blue and red together in the displays. The two
colors are at the opposite extremes of the visual spectrum. Hard to focus on
both. IMHO, that's an ergonomic faux-pas by VW.
Regards
Ralph W. Lambrecht
http://www.darkroomagic.com
Ralph, do you wear glasses? I am curious about this because I have had problems with blue being out of focus at night due to a wrong prescription for corrective lenses. For some time blue neon signs and similar sources looked blurred. When I switched ODs and got glasses which were right this effect went away.
I find I can get well focused prints using either white light or focusing with variable contrast filters in place. I use a simple grain focuser called a Magnasite, it has about a 20X eyepiece in it, evidently a simple achromat.
I can see the chromatic in camera lenses using a very high quality magnifier and looking at either the ground glass or the aerial image. The difference between old Tessars and an Apochromatic Artar are quite obvious. There is fringing visible on sharply defined highlights as the focus moved through the sharpest point on many lenses.
There is no question that some enlarging lenses probably have serious chromatic in the UV region, however, one wonders if there is enough UV available from common enlarging lamps to result in a problem. The Kodak warning applied to a particular lens of very old design.
It was pretty common in the period from the beginning of photoraphy until perhaps the mid-1920's to speak of visual vs: chemical focus. This was partly due to many lenses having poor achromatism in the blue-UV region and partly from the use of photographic materials primarily sensitive to blue and UV light.
FWIW, I use Schneider Compon-S lenses for all but 2-1/4 x 2-1/4 where I have an older Componon and also a Kodak Enlarging Ektar. I can't tell much difference between these two but use the Componon mostly.
--- Richard Knoppow Los Angeles, CA, USA dickburk@xxxxxxxxxxxxx
