I want to discuss the specific technical challenge of painting eyes in different lighting conditions, because eyes are so sensitive to lighting that incorrect treatment immediately reads as wrong.

Eyes in high contrast direct sunlight: the shadow from the brow ridge falls across the upper eye. The iris often shows the catchlight from the overhead sun (typically a small, very bright point in the upper portion of the iris). The whites of the eye may show a shadow from the upper lid.

Eyes in overcast or diffuse light: the shadow from the brow is soft or absent. The catchlight is a wide, soft highlight reflecting the entire sky rather than a point. The whites of the eye appear lighter because there's less directional shadow from the upper lid.

Eyes in interior or artificial light: the catchlight position changes to reflect the lamp or window. The color of the catchlight carries the color temperature of the light source. Eyes in warm interior light have warm catchlights; eyes near a window have cool catchlights from the sky.