My own eye
A macro photograph of my own eye replaced every other reference. From then on the question was no longer whether the render looked like an eye, but whether it looked like this one.
The benchmark. Dark brown iris, almost black, with a blue-grey haze at its edge. A thick lower rim. A low, moist inner corner. Skin with an oily sheen broken into pore-sized highlights.
Why a photograph of me
The early references were stock photographs of strangers. They taught anatomy, and they could not be published. They also pulled in five directions at once, because they were five different people.
One photograph of one eye ends the argument. It also makes the piece honest: the eye that greets you on this site is modelled on the person who made it.
What it was used for
Shape and surface only. The photograph sets where the corners sit, how thick the rims are, how the brow meets the lid. It never sets the light. The light is always the page.
Everything read from it was read on a millimetre grid.