[pure-silver] Re: Chromatic Aberration of Enlarging Lenses
- From: "Tim Rudman" <tim@xxxxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Thu, 27 Apr 2006 10:03:12 +0100
My Peak focus finder has a blue filter for this reason. I was never
convinced that it made any visible difference. I know others who dismiss the
idea (in print) and some who swear by it.
Tim
-----Original Message-----
From: pure-silver-bounce@xxxxxxxxxxxxx
[mailto:pure-silver-bounce@xxxxxxxxxxxxx] On Behalf Of Steve Nicholls
Sent: 27 April 2006 03:17
To: pure-silver@xxxxxxxxxxxxx
Subject: [pure-silver] Re: Chromatic Aberration of Enlarging Lenses
Richard Knoppow wrote:
> 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
>
Richard, I was testing film in my enlarger with an 80A blue filter to
correct the normal yellow of the tungsten bulb exposed to a step wedge and
went back to producing prints and by pure accident left the filter sitting
on the back of the enlarger lens.
The filter had little or no effect on the actual contrast as determined by
the multi grade filters but did result in a "sharper" image.
I experimented by focusing with and without the blue filter behind the
Componon 150mm lens and there was a definite shift in focus. I also tried
another 135mm lens, also of very good quality with similar results.
This is now a practice I have kept.
Steve
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