I'm not going to post any renders until I have some decent textures and surfaces set up, but I made this  eye and did some tests with just colors, no real materials or shaders aside from reflection and refraction. I also made a morph target for it so I can switch between a concave iris and a convex iris. The screen grab here is the "dish" shape, which is the starting default shape.

It may be different when I have textures to apply to it, but the dish and concave shapes seem to make a more realistic looking image when using refraction. That makes sense, because the look of refraction through the cornea IS that of a convex iris.

I was using Lightwave for my tests. I really don't want to get in on the argument about what's best for use in Poser without refraction and raytracing. But if something such as Les's convex morphs can give the illusion of being convex without having to suffer through Poser raytracing, that has to be a good thing. ;-)