I included the screen size, because it's a proxy for working distance, and thus pixel density per viewing angle.

http://punchingin.com/chasing-the-6-4-megapixel-unicorn/

http://www.codinghorror.com/blog/2012/03/welcome-to-the-post...

iPad is "retina" at a viewing distance of 15 inches. iPhone is "retina" at a viewing distance of 10 inches.

Pixel density is only half the equation.