Wasn't it William Tracy who wrote:
>Randal L. Schwartz wrote:
>> So what is the answer?  Can you make a Prism that works, breaking
>> white light into a rainbow?
>
>http://www.povray.org/documentation/view/3.6.1/424/


There's two different effects, both called "dispersion" in POVRay. 
There's the one that makes the pretty patterns in the shadows of sparkly 
things (using photon mapping) and there's the one that causes the 
"chromatic aberration" problem that plagues telescope makers.

I suppose that in the real world they're both caused by the same 
fundamental physics, but in POVRay the settings for enabling the effect 
in the forward mapping of photons is separate from the settings for 
enabling the effect in the conventional tracing of rays.


I vaguely seem to remember that the documentation for DKBTrace claimed 
that chromatic aberration was "a bug in the real world" and was 
therefore not then supported.

-- 
Mike Williams
Gentleman of Leisure

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