There is a book of which I have a copy (now out of print) by the late Canadian Photographer, Gene Nocon. He goes into this focussing lark in some depth and recommends the use of a blue filter over the focussing magnifyer eyepiece lens because the light from the enlarger bulb tends towards the red end of the spectrum and the paper is at the blue end. Using a blue filter over the lens allows the light rays to focus on the actual paper which is blue sensitive as well.
He is using a Peak focus finer in the book illustrations and this has a blue clip on filter for the eyepiece. I do find that it does make a very slight difference so I use the same technique, but calibrate my diachronic filtration in the LPL enlarger head to give me a blue light for focussing. I suppose if you are well off enough to afford an APO lens this does not matter.
Last edited by BMbikerider; 10-22-2012 at 10:06 AM. Click to view previous post history.