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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.

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    $\begingroup$ I'm not sure I understand the question. When I searched for diagrams of the iris, I usually saw cuts of the eye in the medial plane. The few ones which showed a frontal view of the eye showed the iris as a regular circle. You presumably draw eyes from a frontal viewpoint, which also tends to show a circular iris. So where is the discrepancy? Can you post an image from a tutorial with a concave iris and a diagram with a convex iris? $\endgroup$ – rumtscho Apr 7 at 14:53
  • $\begingroup$ I didn't really say anything about at which angle I'd draw the iris, because it's not very relevant. I said "most drawing and 3d modeling tutorial show the iris to be concave", meaning that regardless of the view angle or the viewpoint, you'll always have to take into account that it is concave. Thus, when you draw, even in front view, remember to to shade as if it is concave; when you model an eyeball, don't forget to make it concave. $\endgroup$ – Vun-Hugh Vaw Apr 8 at 9:16
  • $\begingroup$ An iris is three-dimensional. A drawing is two-dimensional, so it represents a cut through the three-dimensional object. For many three-dimensional objects, some cuts are concave and others are convex within the plane of the cut. So it makes the question clearer to explain in which plane you are looking for convexity or concavity. $\endgroup$ – rumtscho Apr 8 at 10:33
  • $\begingroup$ @rumtscho By "concave" and "convex" I simply mean the general tendency, nothing too deep into the weeds of whether some cuts of it could be concave and others are convex. All I care about is whether the iris tends to poke forward toward the cornea ("convex") or recede backward away from it ("concave"), and all that seems to have been easily understood by the answerer below. We could argue all day about how 3D objects manifest in space, but I think the general idea of the question is clear enough. $\endgroup$ – Vun-Hugh Vaw Apr 8 at 11:51
  • $\begingroup$ Thank you! I came up with several different definitions of "convex" and "concave" that would be consistent with your question, and didn't know which is the right one. Funnily enough, it turns out none was the one you had in mind - the explanation of poke forward to the cornea/recede backward is exactly what I was missing. $\endgroup$ – rumtscho Apr 8 at 13:52
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Iris curvature seems to be dependent on the state and health of the eyes.

See Prevention of blinking alters iris configuration in pigment dispersion syndrome and in normal eyes :

Initial iris configuration was concave in all eyes with pigment dispersion syndrome, whereas in control eyes it was concave in four eyes, planar in four eyes, and convex in two eyes.

Other resources :

Accommodation-induced changes in iris curvature

Ultrasound biomicroscopic analysis of iris profile changes with accommodation in pigmentary glaucoma and relationship to age

Quantification of Iris Concavity

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