Is the human iris concave or convex?
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