This is a confusion of two things. Ctein, a writer on photography
some time ago claimed that there was enough chromatic aberration in
common enlarging lenses to cause a focus shift between visual focus
and UV focus. This has proven not to be true. Ctein's results can not
be duplicated by others who have tried.
   It is true that cold light heads have a relatively large amount of
long wavelength UV. It is also true that _all_ photographic emulsions
are sensitive to near UV. However it is likely that most lenses block
so much of the UV light that nothing much beyond visual violet gets
through. Also, modern enlarging lenses are extremely well corrected
for chromatic aberration.
   Graded enlarging papers are mostly sensitive to blue light. VC
paper is also sensitive to green. All are sensitive to UV but as
stated above little gets to the paper.
   Since the eye has a relatively large amount of chromatic
aberration the use of a blue filter for focusing an enlarger is more
likely to result in an error than using white light.
    There are many factors which can result in blurry enlargments
from good sharp negatives. Movement of negatives as they heat up
(popping) is very common. Simple bowing of the negative in the holder
is another problem. The cure for both of the above is to use a glass
sandwich negative holder. The disadvantage of course is keeping four
glass surfaces free of dust.
    Vibration is a common cause of unsharp prints. Its often not
thought of and very little will cause a noticable blur. Alignment of
the enlarger is also important. Film, lens, and paper must be
accurately parallel for best results.
    Good enlarging lenses are, of course, key. There are poor ones
but even the older top of the line lenses are quite good. The lens
must be very clean. Haze or lens damage will lower print contrast and
can also result in blur.
    I may have left something out but the type of light source is NOT
a factor in sharpness.

Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com