Blue focusing filter on grain focuser

[uskanb2n@ibmmail.com]

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...

                                        - John Lowther.