I have a pretty typical glass-sphere-on-grey-floor scene,
and I'm using dispersion and photons in it.  
I'm using Nathan Kopp's color map macro on three spotlight sources.
My questions:

The shadow of the sphere starts out black at the edge, stays black for a
medium
distance, and then the colors and bright spot from the dispersion begins.
What do I change so that the black shadow is brighter or starts to fade
into the bright spot more thoroughly (i.e. closer to the shadow's edge?) . 
I guess what I'm looking for is an increase in caustics around the base,
but without messing up the prism effect too much.

Second, I've played with the disp_nelems for both the light source macro
and the target sphere but I still see color banding.  Which do I need to
raise the nelems on to eliminate it (I've gone up to 20 so far) and how
high should it be to look nice at, say, 640x480?

Lastly, the ground surface is flat, but I get all sorts of tiny, regular
streaks in the
dispersion highlight.  For some reason, this looks right to me, but I want
to know
where it's coming from - the roughness on the sphere (.005) or the photons.

I know I should trial-and-error this stuff myself, but my computer is
pretty slow.

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