How many megapixels are your eyes?

UPDATED based on comments from Martin Robbins: The latest marketing nonsense from that computing company with the chewed fruit logo is grabbing the attention of the blogosphere for its claims that the iPhone 4 has higher resolution than is discernible by the human eye in its so-called “retina” display. So, can that possibly be true? Down at the cellular scale of rods and cones, the resolution must surely be way beyond any LED, regardless of whether it’s labeled organic.

So, how many pixels are your eyes? A quick Google turns up a nice article that compares the human eye with a camera:

“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. [There is also the issue of saccades and what happens during those] 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.”

The following calculation shows the equivalent megapixel number of the human eye viewing a scene.

“Consider a view in front of you that is 90 degrees by 90 degrees, like looking through an open window at a scene. The number of pixels would be 90 degrees * 60 arc-minutes/degree * 1/0.3 * 90 * 60 * 1/0.3 = 324,000,000 pixels (324 megapixels). At any one moment, you actually do not perceive that many pixels, but your eye moves around the scene to see all the detail you want. But the human eye really sees a larger field of view, close to 180 degrees. Let’s be conservative and use 120 degrees for the field of view. Then we would see 120 * 120 * 60 * 60 / (0.3 * 0.3) = 576 megapixels.”

The iPhone 4 resolution on its 3.5″ display is 960×640. That’s quite a lot less than one megapixel, but it’s in a much smaller area than one’s field of view at a normal viewing distance. Obviously, you don’t fill your field of vision with the screen at normal distances. I expect most people with half-decent sight will hold a phone at about 12 inches away, so that’s probably about 7 iPhones high and maybe 12 wide, so very approximately 100 iPhones would fill you field of view, 100 megapixels, give or take.

But, Martin points out that actual human limit is about ~480 pixels per inch at 12 inches, or ~320ppi at 18″ (arms length) and Apple marketing claim is for arm’s length. At that distance field of view would probably be an array of about 500 iPhones…which would be roughly 500 megapixels…which is roughly what we worked out the pixel count of one’s eyes to be earlier on…so, yes. Seems like the retina display could just live up to the hype. Darn it.