Re: Blue focusing filter on grain focuser

[George Huczek <ghuczek@sk.sympatico.ca>]

At 10:50 AM 10/10/97 EDT, you wrote:
>
>Seems like a few months ago Photo Techniques ran an article by a NASA
>scientist which showed that chromatic aberration of the EYE rather than
>the enlarger lens was the major culprit in Ctien's out of focus
>enlargements on VC paper: The point of focus determined through a
>magenta filter for low contrast was quite removed from that determined
>with white or blue light. There was some shift between white and blue,
>so the conclusion was to focus with white light to get the best focus
>for VC material.
>
>With graded paper, a blue filter might, just possibly, have a place
>since graded paper is sensitive only to blue light...
>
>
I read the article, but I'm not really convinced by any of this, unless one
is focusing with the enlarging lens stopped down at the aperture with which
the paper will be exposed.
   I open my aperture fully when focusing, because for me it is more
important to have a bright grain pattern on which to focus with a critical
enlarging focuser.  Stopping down the lens, then adding a filter
(especially a blue one) on the eyepiece of the focuser, plus the extra
density of contrast filters with multigrade papers, makes the grain pattern
so dim that I am likely to have more problems obtaining critical focus due
to the _dimness_ of the light rather than the fact that the wavelength is
off by 100 nanometres or so.  After focusing wide open, with white light,
and with all filtration removed from the enlarger, I then return the filter
pack and stop down the lens.  Even with the limited depth of field of
enlarging lenses, I yet have to be convinced that the increase in depth
after stopping down does not adequately compensate for the focus shift due
to the colour of the focusing light.  It seems to be one of those overly
technical matters which does not seem to translate in actual working
practice.  

- -GH