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Dr. Raymond Soneira runs DisplayMate Technologies, which makes software to test display quality. He also knows more about digital displays than just about anyone I know - and I know some pretty tech-savvy folks. This morning, Dr. Soneira shot me an interesting email regarding the so-called "Retina Display" of the iPhone 4. To clarify: a retina display is one whose resolution meets or exceeds the maximum resolution the human retina is capable of resolving, assuming perfect vision.
This is a bit tricky, since the eye doesn't have "pixels" and the resolution required to match the human eye's capability depends on the distance from your eye to the display. If you sit four feet away from a 50" 1080p television, you'll see pixels. If you sit 100 feet away, you won't. The distance between any two visual elements is a matter of how many pixels per "arc degree" of vision it covers. Dr. Soniera's email, in full and unedited, is as follows:
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The iPhone 4 has an outstanding display... and I'm glad that Apple resisted the emotional rush to OLEDs because they still need lots of improvement before they will be ready to compete with the highly refined IPS LCDs. The iPhone 4 display should be comparable to the outstanding IPS LCD in the Motorola Droid, which I tested and compared to the Nexus One OLED, which was trounced by the Droid.
Steve Jobs claimed that the iPhone 4 has a resolution higher than the retina - that's not right:

1. The resolution of the retina is in angular measure - it's 50 Cycles Per Degree. A cycle is a line pair, which is two pixels, so the angular resolution of the eye is 0.6 arc minutes per pixel.

2. So if you hold an iPhone at the typical 12 inches from your eyes, that works out to 477 pixels per inch. At 8 inches it's 716 ppi. You have to hold it out 18 inches before it falls to 318 ppi.

So the iPhone has significantly lower resolution than the retina. It actually needs a resolution significantly higher than the retina in order to deliver an image that appears perfect to the retina.


It's a great display, most likely the best mobile display in production (and I can't wait to test it) but this is another example of spec exaggeration.

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So there you have it - some math from a display expert showing that, while the iPhone 4's display is certainly exciting and probably represents a step forward for smartphones, it may fall short of Apple's claims of meeting or exceeding the resolution of the human retina.
Be sure to check out our other coverage of cell phones.