Most drawing and 3D modeling tutorials show the iris to be concave. But all the diagrams I Googled show the iris is at best flat, otherwise even a little convex (discounting the optical distortion in photographs). So is it concave or convex?
Edit: Just to be clear, the question only concerns the concavity of the iris. The reason I mention drawing and 3D modeling here is that because it's such a common practice to make the iris explicitly concave in a somewhat exaggerated way, which is at odds with countless profile diagrams of the eye that I could google.
For example, this video (at about 5:52) even says that "it is convex according to anatomical diagrams, but I like to draw it as if it is concave". Or in this video (at about 7:46), the modeler pushed the concavity of the iris quite far, it's like a deep indent.