That is a great article that even gives you a formula for calculating it, which by the way was featured on one of apples slides during the presentation. Retina

That is a great article that even gives you a formula for calculating it, which by the way was featured on one of apples slides during the presentation. Retina is a function of "normal" distance from face and "normal" eyesight. Those are the only two degrees of freedom. I am not sure why the author chose to guess his own numbers to see if they compare with Apples, I would instead take the formula and the fact that iPhone 4 is definitely Retina and figure out the appropriate Retina value.