Apple makes a pretty ballsy claim about the iPhone 4's display: "The pixel density is so high that the human eye is unable to distinguish individual pixels." They're pushing against the limits of the human body. Is it true?
The iPhone 4's 3.5-inch display is the highest resolution screen ever put in a phone. It's stuffed with 614,400 78 micrometer-wide pixels, for a pixel density of 326 pixels per inch. Those are the facts, according to Apple.
What's disputed is whether or not the iPhone 4 possesses a true retina display, one in which the human eye can't see individual pixels.
1. The resolution of the retina is in angular measure - it's 50 Cycles Per Degree. A cycle is a line pair, which is two pixels, so the angular resolution of the eye is 0.6 arc minutes per pixel.
2. So, if you hold an iPhone at the typical 12 inches from your eyes that works out to 477 pixels per inch. At 8 inches it's 716 ppi. You have to hold it out 18 inches before it falls to 318 ppi.
So the iPhone has significantly lower resolution than the retina. It actually needs a resolution significantly higher than the retina in order to deliver an image that appears perfect to the retina.
At 12 inches the 1 dimensional linear difference is 326/477 = 68 percent. But the pixel (area) density for 2 dimensions, which is the actual relevant observable, is that value squared = 0.47, so the iPhone 4 is more than a factor of 2 from being a retina display at the typical 12 inch viewing distance. Stated another way the iPhone display would need to have 1.3 megapixels instead of 0.6 megapixels to be a retina display.
Okay, got all that? On the other hand, William H.A. Beaudot, president of KyberVision tells The Loop that "Apple's claim is not just marketing, it is actually quite accurate based on a 20/20 visual acuity." Here's what he says in detail:
"A visual acuity of 20/20 means that a normal human eye can discriminate two points separated by 1 arc minute (1/60 deg). A visual angle of 1 arc minute seen from a distance of 1 foot corresponds to a dot size of about 89 micrometers or a pixel density of 286.5 dpi. Since the 'retina' display has a pixel density of 326 dpi (14% better than what we would expect from a 20/20 visual acuity at 1 ft), it would seem unfair and misleading to refute Apple's marketing claim on this basis.
Since this display is able to provide a visual input to the retina with a spatial frequency up to 50 cycles per degree when viewed from a distance of 18-inches, it almost matches the retina resolution according to the Nyquist-Shannon sampling theorem."
Popular Mechanics courts the opinion of Ethan Rossi from the Center for Visual Science at the University of Rochester who says the iPhone 4 "surpasses 20/20 human visual capabilities" at 16 inches and 12 inches away.
It's when you get within 3 inches that he says you'll be able to see the pixels. Ethan wrote to clarify: "At about 25 cm (10 inches) is when the pixels will be larger than 1 arcmin, which is the equivalent to 20/20 vision."
Hokay, well. The one thing they all definitely agree on is that it ultimately comes down to how far you hold the iPhone from your face. The further away, the better it's going to look.