[pure-silver] Re: Chromatic Aberration of Enlarging Lenses

Richard

I do for a few years now. But the problem is worse without them.





Regards



Ralph W. Lambrecht

http://www.darkroomagic.com







On 2006-04-28 01:55, "Richard Knoppow" <dickburk@xxxxxxxxxxxxx> wrote:

> 
> ----- 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
> 
> 
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