Starting with the definition from Wikipedia:
The human eye is an organ that reacts to light and has several purposes. As a conscious sense organ, the mammalian eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including colour differentiation and the perception of depth. The human eye can distinguish about 10 million colours
Similar to the eyes of other mammals, the human eye's non-image-forming photosensitive ganglion cells in the retina receive light signals which affect adjustment of the size of the pupil, regulation and suppression of the hormone melatonin and entrainment of the body clock.
You might, now, ask how many megapixels a human eye has. Well even if you didn’t, here's the answer.
The average human retina has five million cone receptors. Since the cones are responsible for colour vision, you might suppose that this equals a five megapixel equivalent for the human eye.
But there are also a hundred million rods that detect monochrome contrast, which plays an important role in the sharpness of the image you see. And even this 105MP is an underestimate because the eye is not a still camera.
You have two eyes (no kidding!) and they continually flicker around to cover a much larger area than your field of view and the composite image is assembled in the brain - not unlike stitching together a panoramic photo. In good light, you can distinguish two fine lines if they are separate by at least 0.6 arc-minutes (0.01.Degrees).
This gives an equivalent pixel size of 0.3 arc-minutes. If you take a conservative 120 degrees as your horizontal field of view and 60 degrees in the vertical plane, this translates to 576 megapixels of available image data!
Curiously - as a counterpoint to this - most people cannot distinguish the difference in quality between a 300dpi and a 150dpi photo when printed at 6x4", when viewed at normal viewing distances.
So: although the human eye and brain when combined can resolve massive amounts of data, for imaging purposes, 150dpi output is more than enough to provide adequate data for us to accept the result as photographic quality.
But don't forget that women have more cones and men have more rods - I kid you not. Therefore the ladies see colours brighter than gents but can't see as well when it gets dark.
Here are 101 astounding facts about the human eye. Some of them are really surprising!
Sources: +Wikipedia +Lenstore +Yahoo!
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