retina

The eye is often described as a camera, and the retina is the part that corresponds to the film or the sensor. The US National Eye Institute puts it plainly: when light hits the retina, a light-sensitive layer of tissue at the back of the eye, special cells called photoreceptors turn the light into electrical signals.

Those signals then travel from the retina through the optic nerve to the brain, and the brain turns them into the images you see. So the retina does not see anything by itself. It converts, and everything a person experiences as vision happens after that conversion, further along the chain.

This is why the retina matters so much in stories about restoring sight. If the lens is cloudy it can be replaced, and if the eye is misshapen the light can be redirected. But if the photoreceptors themselves have died, there is nothing left to do the converting, and no amount of light reaching the back of the eye will produce a signal. That is the gap a retinal implant is built to bridge: it sits at the same place in the chain and generates the electrical signal that the missing cells no longer can.