Archived posting to the Leica Users Group, 2000/02/28

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Subject: Re: [Leica] Re: APO for B & W??
From: "Tom Schofield" <tdschofield@email.msn.com>
Date: Mon, 28 Feb 2000 10:38:12 -0800

Doug's response is consistent with my understanding.  Poor correction of
chromatic aberrations leads to color fringing.  It can be even worse in B&W
than color because the white light coming through the negative contains all
colors of the spectrum, whereas the color neg or slide filters some of the
colors out, so the fringing is less noticeable.  On B&W the "color fringing"
bears out as unsharpness or loss of acutance, rather than seeing the colors
in the color firinging.

Tom Schofield

- ----- Original Message -----
From: "Doug Herr" <Dherr@energy.state.ca.us>
To: <leica-users@mejac.palo-alto.ca.us>
Sent: Monday, February 28, 2000 9:49 AM
Subject: [Leica] Re: APO for B & W??


>
>
> Charles Cason <cec@vbe.com> 02/28/00 08:34AM wrote:
> >>>
> I'm not sure I understand all the questions about APO lens.  I think an
> APO lens will not resolve any more lines than a non APO, only effect the
> chromatic color scale???
> If you are developing black and white, does it matter if the enlarging
> lens is APO or not?  Can someone, in a few words, correct my thinking on
> this.
> Thanks Charles
> <<<
>
> Unless your light source is a single wavelength, light is color whether
the film or paper records separate wavelengths or records intensity (as in B
& W).  You want all wavelengths to focus at the same place otherwise you get
a blur instead of a point.  An APO lens is defines as one where 3
wavelengths (I forget which 3) at various points in the spectrum focus at
the same point.  Non-APO lenses don't nessesarily do this.
>
> Doug Herr
> Sacramento
> http://ourworld.compuserve.com/homepages/telyt
>
>