Evolution of half an eye right before your eyes:
This is the vent shrimp - rimicaris exoculata - which, as adults, live in the abyssal depth near hydrothermal vents, where it is almost completely dark. The infinitesimal amount of light available there are created by chemical reaction between minerals in the hot vent water, and are far too dim to be captured by the aperture for any camera eye or compound normal eye of any other creature.
Rimicaris larvae swim in comparatively well lit shallow depth and are thus born with normal shrimp compound eyes. When they mature they sink down into the abyss near hydrothermal vents where their normal eyes become useless, due to the dark, so the eyes are completely reabsorbed when the shrimp reach adulthood. As the picture shows, the adult rimicaris does not have any eyes.
Except not quite. The normal shrimp eyes are totally gone. But notice the patch of brown discoloration on the shrimp's back. The adult rimicaris evolved a totally new form of eye, not seen in any other animal, that is really, quite literally, half an eye. After it absorbs its normal shrimp eyes, rimicaris developed, get this, a naked, totally unprotected retina, on the back of its shell. That's brown discoloration.
It sees with its shell, literally. It is as if you took a normal eyeball, cut away the pupil and the lens, take the remaining backhalf of the eyeball, and lay it out flat on the shrimp's shell. This precisely how the brown patch, the retina on the back of rimicaris, works.
Why does it work this way? Because normal eye lacks sufficient aperture to capture enough of the few photons down at that depth to be useful. So the shrimp evolved a new eye with the maximum possible aperture, which is no aperture at all, Ie, a naked retina spread out flat to avoid castinga any shadows so as to capture every last stray photon. Half an eye, retina only, no lens, no pupil.
Half an eye, ideally adapted to the shrimp's environment.
Creationists who say the eye is irreducibly complex because anything less than a complete eye is useless, you can eat shit and die.