I want to share what I know about painting realistic eyes, because eyes are the element that most often determines whether a portrait reads as alive or flat.

The eye as a sphere: the eyeball is a sphere sitting in a socket. The visible portion — the iris, pupil, and surrounding white — is a flat disc on the surface of that sphere. The eyelids wrap around the sphere. This geometry produces the characteristic highlight at the top of the iris (from the upper eyelid's shadow cast on the spherical surface) and the slightly lighter value at the bottom of the white (reflected from the cheek).

The catchlight: a small bright highlight that reflects the light source. Without a catchlight, eyes look flat. The catchlight's position tells the viewer exactly where the light source is. Consistency across both eyes is critical — inconsistent catchlights look wrong.

The detail hierarchy: the iris has texture (radial strands) that you should suggest without over-rendering. The pupil is pure dark at the center. The sclera (white) is almost never pure white — it has the color of the ambient light and the slightly warm pinkish color of the tissue.