The iPhone 4 has a sweet-sounding "Retina Display", so called because it has a higher pixel density than the 300 pixels-per-inch (PPI) the human eye is able to discern. Take your calculators out boys and girls, let's see if this is true! It's a very simple calculation.
iPhone 4 has 326 PPI and 1 inch = 25.4 mm
1 pixel is 25.4/326 = 0.0779 mm
The human eye has an angular resolution of 0.02°. What is angular resolution? It's no use saying how big a thing the eye can see, since that depends on how far away it is. Angular resolution takes the distance into account and is the angle between 2 objects and your eye.
Now let's see how far away 0.0779 mm must be to have an angular separation of 0.02° (that text replacement is coming in useful)
Imagine a right-angled triangle with one side length x and the perpendicular side length 0.0779/2 mm. Why /2? Well, the distance is measured to the centre of 2 pixels, and the distance from the centre to either pixel is half the pixel spacing. We want the angle to be 0.02/2°.
tan 0.02/2° = 0.0779/2/x
x = 223 mm ~ 22 cm
That means, you must hold the display closer than 22 cm from your eye to be able to distinguish the pixels.
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