[QUOTE=dr bob] I made the mistake of thinking a Wratten 11 or 15
would bring out the "sparkles" in a local scene - wrong! The
result was completely dark shadows without any detail (TX400). I
rephotographed with a Wratten 47 (blue) and the shadows popped
out perfectly. This seems to fly in the face of general logic
until one considers that the shadows in a sunlit scene are
illuminated by blue light.   Dr. Bob it seems to me that your
stated experience verifies that shadows are illuminated with
blue light. Yellow filtration would be minus blue and that would
account for the deepening of shadow values that you noted. The
47 blue filter is plus blue and would account for the lightening
of shadow values that you indicated.  This blue filter would
have the effect of lowering local contrast within the snow
itself since local contrast within the snow itself would contain
small shadow areas that are lit by the same blue light that you
noted in the shadows.  While a full scale scene may have shadow
and snow both includ