Iris photography is pretty finicky. The best eye photos are high resolution (these are 24 Megapixel), show mostly eye and very little face, are well-illuminated, have little if any room reflections, and utilize frontal and side-lighting options to help us see specific details.
Each eye needs a minimum of seven images as you see below.
A complete set of eye photos includes images of each sclera. When photographing the sclera it is important to use less light. Too much light will wash out the appearance of the blood vessels and other sclera markings.
Frontal lighting for eye photos can be best achieved using a camera that has two light sources that can be fired off simultaneously, or individually for side lighting.
The cornea protrudes slightly from the front surface of the iris. When using side lighting the opposite side of the iris will appear in shadow. Notice how side lighting makes certain features, such as contraction furrows, more obvious.