I’ve modelled this eye. The iris is a separate object. The schlera overlaps the iris and should be casting a shadow on it, but doesn’t. The eyeball texture is a blend with the end of the colorband set to transparent where the cornea occurs. I have set the iris material to receive transparent shadows, and I’m using raytracing. So, any suggestions as to why there isn’t a ring of shadow around the outside edge of the iris? Lamp is a spot and casts shadows elsewhere. Textures are not final - I know they don’t look so great at the moment.
ETA I’ve just had a thought. The eyeball material is set to transparent. The opacity of the schlera is provided by the blend texture. Does Blender recognize this, for shadow casting purposes?
