Archived posting to the Leica Users Group, 1999/02/02

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Subject: Re: [Leica] Enlarging lens
From: "Martin V. Howard" <marho@ida.liu.se>
Date: Tue, 02 Feb 1999 12:34:36 +0100

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