Apple’s “Retina Display”: What Does It Mean?

A major point in the hoopla yesterday about the iPhone 4 was Apple’s introduction of the “Retina Display”
To quote from Apple:

Text in books, web pages, and email is crisp at any size. Images in movies and photos are stunning at almost any angle. That’s because the Retina display’s pixel density is so high, your eye is unable to distinguish individual pixels.
In a word, resolutionary.
By developing pixels a mere 78 micrometers wide, Apple engineers were able to pack four times the number of pixels into the same 3.5-inch (diagonal) screen found on earlier iPhone models. The resulting pixel density of iPhone 4 — 326 pixels per inch — makes text and graphics look smooth and continuous at any size.

So are they right? Or is this just another way for Apple marketing to get us to buy a shiny thing? (Spoiler: Probably yes to both)
The maximum spatial resolution (ability to differentiate two points) of the human eye occurs at the center of the visual field, corresponding to the fovea of the retina. At the fovea, the cone cells (there are no rods in the fovea) are jammed up close together at the highest density of the retina. Knowing this we can calculate the smallest pixels the fovea should be able to differentiate, but it takes a couple assumptions that we will lay out. The reader is encouraged to re-calculate the following based on their affinity for Apple.
1) Assuming 20/20 vision with bright lights and adequate contrast, the fovea has the ability to differentiate points around 1 arc-minute apart.
2) Assuming the user holds the phone at 1 foot (0.305 m) from their eyes, 1 arc-minute corresponds to 89 micrometers.
3) Apple’s claim of 78 micrometers is smaller than 89 micrometers, however 2 points are needed to differentiate, so that is 78 x 2 = 156 micrometers, which is not.
Also, depending on the literature, there are claims the fovea can determine points 0.5 arc-minute apart, which is 44 micrometers. Regardless, it is close, and the user is unlikely to always be in perfect light or always using foveal vision and resolution drops off pretty dramatically a very short distance along the retina from the fovea.

Apple: Retina Display…

Angular Size Calculator…

University of Utah: Visual Acuity…