I was trying to fine tune some photon media parameters (in global_settings), and
tried varying the factor.  There seems to be no visible effect at all, although
the mere inclusion of the parameter doubles or quadruples the render time.  Am
I missing anything?  Am I just using it incorrectly?

Below is a code extract, run with version 3.6.1 with +kff7 and +kff-1.  (I set
the max_steps value low in order to contrast the effect of the factor
parameter.)  CPU time averaged 493 seconds per frame vs 106 seconds without the
factor parameter.  All 8 images are virtually identical.

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#if (clock_on) #declare Factor = pow (2, frame_number - 4); #end
#declare R = 0.05;
#declare W = 8;
#declare H = 6;
#declare S = 0.005;
#declare M = 50;
#declare Margin = 1;
#declare v_Source = <W/2 - Margin, Margin - H/2, 0>;
#declare v_Target = <Margin - W/2, 0, 0>;

global_settings
{  assumed_gamma 1
   photons
   {  spacing S
      media M #ifdef (Factor) , Factor #end
   }
}

camera
{  orthographic
   location -z
   up H * y
   right W * x
}

box
{  -1, 1 scale <W/2, H/2, R> * 1.001
   hollow
   pigment { rgbt 1 }
   interior { media { scattering { 1, 1 extinction 0.001 } } }
   photons { pass_through collect off }
}

box
{  <v_Target.x, -0.1 * H, -R>, <v_Target.x - Margin/2, 0.1 * H, R>
   pigment { rgb 1 }
   finish { reflection 1 diffuse 0 ambient 0 }
   photons { target reflection on collect off }
}

light_source
{  v_Source, rgb 0.5 / R
   cylinder radius R/2 falloff R point_at v_Target
   parallel point_at v_Target
}

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