Raybias too high.

I can't explain why the renderer has to work this way. Makes no sense to me. All I know is given the point from which you are ray-tracing a refraction, any geometry closer than the ray bias is ignored. You are getting the back of his head, not his iris, because the iris is closer than the ray bias.

You said even if you use low raybias there is still a hard edge. That is correct. The person who modeled the cornea made it coincide with the edge of the iris instead of being a layer above it. This is where physics matters. A real cornea surface is never touching the iris - it is above it. But in the model, they touch, i.e. the distance is 0. So you can never set a ray bias low enough.

Instead of decreasing ray bias, you can try to apply a tiny displacement to the cornea and eye surface. Do not try a negative displacement on the iris. Poser does not "see" displacement too well when performing a refraction or reflection, so if you move the iris, it will still appear as if not moved and the raybias problem will remain. On the other hand, in Poser the origin of a refraction can be displaced so moving the cornea might help.

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