Most modern high-quality enlarging lenses focus blue and near-UV in the same plane as red and green, or at least close enough for enlarging purposes. Older and uncorrected lenses often focus blue (to which your paper is sensitive) in a slightly different plane than red/green (to which your eyes are most sensitive). For these uncorrected lenses, using the blue filter will get you better focus. For anything else, it is superfluous.

Do a quick test: focus with and without the blue filter, make a couple of test strips/prints and see if the blue filter gives you better results. I doubt it will, especially if you are using VC paper, which is also sensitive way into the green part of the spectrum.

I've never used my blue filter with my Peak grain focusers, and have no complaints about the sharpness of my prints.

Best,

Doremus


www.DoremusScudder.com