radius controls the maximum distance within which Mental Ray
considers a photon to compute the caustic and determines how much of
the effect is illuminated.
The radius value can be either
specified or estimated automatically by Mental Ray based on the size
of the bounding box of the scene. The estimated value will be used
in the render if a value of zero is entered in the radius value
option. Therefore the default appears as zero but is actually a
Increasing the radius value effectively
increases the size of the effect so larger values will produce
larger caustics but some detail will be lost in the results.
It is best to adjust the value by small amounts. A value of
0.05 can be used to show the individual photons used to create the
effect. Larger radius values will illuminate more of the caustic
effect and can reduce grainiess but will diffuse the light and cause
the effect to appear dimmer.