Natalie Angier in The New York Times:
If you’ve ever stumbled your way through a newly darkened movie theater, unable to distinguish an armrest from a splayed leg or a draped coat from a child’s head, you may well question some of the design features of the human visual system. Sure, we can see lots of colors during the day, but turn down the lights and, well, did you know that a large bucket of popcorn can accommodate an entire woman’s shoe without tipping over?
Yet for all these apparent flaws, the basic building blocks of human eyesight turn out to be practically perfect. Scientists have learned that the fundamental units of vision, the photoreceptor cells that carpet the retinal tissue of the eye and respond to light, are not just good or great or phabulous at their job. They are not merely exceptionally impressive by the standards of biology, with whatever slop and wiggle room the animate category implies. Photoreceptors operate at the outermost boundary allowed by the laws of physics, which means they are as good as they can be, period. Each one is designed to detect and respond to single photons of light — the smallest possible packages in which light comes wrapped.