What Retina Display Actually Means, and What You Need to Know about PPI
Retina display does not match our retina resolution, but it serves as an excellent approximation and marketing term.
Retina display was first introduced in an Apple event back in 2010. As Steve elaborated during the iPhone 4 event, the pixels become invisible to the human eye when you hold your device 10 or 12 inches away from your eyes and the pixel density exceeds 300 PPI(pixels per inch). I made some simple calculations based on trigonometric functions, and the statement can be simplified to the following. When every pixel only counts for 1 arcmin(1/60 degrees) or less to your eyes, you will not see any individual pixel but a continuous surface.
However, in reality, that’s only correct for some people, not all of us. When you hold your device 10 or 12 inches away from your eyes, sometimes you can still see individual pixels. That’s because the angular resolution of our retina is actually higher than 1 arcmin, then why Steve told us that you could not see individual pixels in this condition. Except for marketing purposes, it is an excellent approximation for most people in most conditions.
