In that case, for many figures, the iris is a distinct material, and the only problem might be where it is on a texturemap. Some figures have it tucked away on the general head texturemap, and the UV mapping means you have to match that. DAZ3D usually have well-labelled templates available. The extra layers, and their transparency, wouldn't need changing from the default for a working figure. If you're doing exotics, you might have to do the whole eye. I've not seen an eye where the UV mapping for pupil/iris/white isn't a single area, and the different parts cannot cross material boundaries. You'd have to do that to get a slit pupil, or just different light levels. Which is a thought -- especially with slit pupils, you get some noticable changes as light level drop. A slit pupil changes shape. Would it be worth putting some index marks outside the texture area to make it easier to identify the eye's centre?