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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.

can you show a primitive painting how th cornea on apollo eyes should be?

i will try to do a morph to fix this.