[rayiner]

Humans can distinguish a lot more than that. See: http://clarkvision.com/imagedetail/eye-resolution.html.

His calculation gives 530 ppi at 20 inches.

Explanation: "The eye is not a single frame snapshot camera. It is more like a video stream. The eye moves rapidly in small angular amounts and continually updates the image in one's brain to "paint" the detail. We also have two eyes, and our brains combine the signals to increase the resolution further. We also typically move our eyes around the scene to gather more information. Because of these factors, the eye plus brain assembles a higher resolution image than possible with the number of photoreceptors in the retina."