Re: [Leica] Enlarging lens

["Joe Stephenson" <joeleica@flash.net>]

Dear Martin,
I appreciated the point you make regarding the inability of b&w paper to
respond to red light, making an apo lens unnecessary. Therefore, I wrote to
Rodenstock with just this question. Here is the reply I received from Don
Nagle:

1st - These questions should be directed to Bob Solomon @ HP Marketing
973-808-9010 because we are not the Photography division. We are the OEM
division for Industrial Optics.

I can tell you that a non-chromatic corrected lens WILL CAUSE unsharpness in
Black & White photos & color fringes in color exposures. Without getting
into Optical theories it basically has to do with the frequency of light
waves as they pass through various types of glass. Black is the abscence of
color and white consists of the entire visible spectrum. Therefore it goes
that white is the defining principal in black $ white photography and
consists of all wavelengths. Therefore, lenses that correct for all 3
primary wavelengths (APO) will increase sharpness in any application whether
it be B&W or Color...

As Don suggests, I will try to follow up with Mr. Solomon and share his
response here.
Sincerely,
Joe Stephenson

- -----Original Message-----
From: Martin V. Howard <marho@ida.liu.se>
To: leica-users@mejac.palo-alto.ca.us <leica-users@mejac.palo-alto.ca.us>
Date: Tuesday, February 02, 1999 5:05 AM
Subject: Re: [Leica] Enlarging lens


>Joe Stephenson wrote:
>>
>> As to the use of apo lenses for black and white. I have read that apo
>> offers advantages in B&W printing because of the apo's lenses ability
>> to focus light of three (rather than two) wavelengths on exactly the
>> same plane. If some wavelengths of light are focused at a point behind
>> or ahead of the film plane, the result will be a fringe or halo around
>> the area of sharp focus produced by the properly focused wavelengths
>> of light. When printing color, the result will be a color fringe
>> around objects. In B&W one will see monochrome fringes.
>>
>
>This doesn't make a whole lot of sense.  B&W paper is not sensitive to
>the whole spectrum, only the bluey-greeny parts of it.  APO lenses
>typically ensure that the red part of the spectrum also focuses on the
>same plane as the blue-green part.  If B&W paper was sensitive to the
>red part of the spectrum, safelights wouldn't work.
>
>Since regular (non-APO) lenses already focus blue-green light in the
>same plane, there is little gain to be had for an APO-lens when printing
>B&W.
>
>Colour, obviously, is another matter.
>
>M.
>
>--
>Martin V. Howard, Application Systems Laboratory,     |
>Dept. of Comp. & Info. Sci., Linkoping University,    | Just "DOHH" it!
>SE-581 83 Linkoping, Sweden.  Tel +46 13 282 421,     +----------------+
>Fax +46 13 142 231; marho@ida.liu.se; www.ida.liu.se/~marho
>