I have set up an optical slit with a lens such that the light line should travel through the lens and (depending on the angle of incidence) bend relative to the refractive index of the lens.
But as you can see from the JPG, light is not traveling through the lens at all.
Refractive (and Reflective) Caustics are on in the render panel. Auto smoothing is on. I can’t think where else to look.
Thanks for any clues.
This is a very hard scenario for Cycles (or any unidirectional path tracer, for that matter). I suggest that instead you try a render engine that uses bidirectional methods, such as RenderMan (with VCM enabled) or LuxRender.
If you’re playing with realistic lenses, LuxRender does a much, much better job.
Thanks to Stefan Werner and ambi. I have taken your advice and now have BlendLuxCore add-on working in my Blender 2.79b.