If we take a look on the render, we can see that we got a nice realistic shadow on iris... but the darker part is not in texture. We have to get it from shadows similar to an real eye. Most of the people put that darker part in texture and which would be not correct. That comes from shadow and we made it right way. We need to turn on refractions on eye shader like we have in real eye and pupil is still a bit too big and has harsh egde and we need to fix that. We’ll make one more ramp, circular ramp for transparency of iris object and turn on refractions on blin shader. Turn on refractions and make index of refraction to 1.33. that is common index for glass, water and such materials. Now let’s reshape the iris a bit. We’ll use cluster for that. Select iris object and go to animation panel, in deformers chose Cluster with iris object selected and you will get C letter in center of iris object. That is cluster. If you try to scale cluster you will scale whole iris object. But we don’t want that! We need to paint cluster weights on iris object and later with that we can animate pupil growing and shrinking like real eye does. Select iris object and from edit deformers choose paint cluster weight tool. When you do that you’ll see that iris is whole in white color. That means cluster have 100% weight on whole iris object. We need to tweak that! We want to make it such that cluster has more strength in center and fades of as it moves away from center.

Use paint tool. Make value 0 and press FLOOD. Whole iris will be black now. That means that there is no influence from cluster. Than change value to 1 and using paint operation REPLACE and paint first row of vertices in white. Change value to .5 and paint next row.. than change value to 0.25 and paint next row. You’ll get nice gradient fall off as shown in the below image.

Now scale your cluster down a bit until you get right size of pupil. Now you have to set pupil rig for eye. Now with this cluster you can animate pupil grow and shrink if your character goes from dark to light environment and so. Like real eye does.