I've stared at my colleagues' eyes over lunch, and I think unless we can get actual refraction of the light entering the cornea - does Poser do that? I'm clueless - we'll need to make the iris somewhat concave to mimic the way it catches the light. Not dish-shaped like they are now, but uniformly concave. Without diffraction, a convex shape is definitely not going to produce the correct image.

Also, like cobaltdream said, the iris needs to be set back a little to catch a shadow at the edges.

My $0.02,

-- I'm not mad at you, just Westphalian.