I’m a pretty big Apple fan. I have Apple computers, an iPhone, an iPod, and an iPad. But yesterday as I watched the coverage of Steve Jobs’ WWDC Keynote, I bristled at the name of the new iPhone 4 display. Jobs called it the Retina Display and seemed to be making the claim that there was no need to make a higher-resolution display because the human eye wouldn’t be able to detect any more pixels.
The new display does sport some impressive specs: 960 X 640 pixels, for a resolution of 326 pixels per inch (ppi). That’s four times the resolution of the previous-generation iPhone, and perhaps 10 times the pixel density of a typical desktop computer display. But the real question is, how does it compare to the human retina?
The answer is a bit more complicated than you might think. The retina isn’t like a computer display—it doesn’t have consistent resolution throughout. Most of the eye, in fact, has quite low resolution. You only see details in a tiny portion of the visual field, defined by a special portion of the retina called the fovea. But since the eye can move, a display needs to match not the average resolution the eye can detect, but its highest resolution—the resolution of the fovea, corresponding to just 1/2 of a visual degree.
The area seen by your fovea is roughly equivalent to the size of your thumbnail at arm’s length. But since your eye is constantly moving, you’re able to construct the illusion of seeing fovea-level detail in your entire visual field. And, in fact, you can see it, because as soon as you focus on a region of a display, your fovea orients to it.
What’s the resolution of the fovea? The best calculation I’ve seen is here. This is the critical bit:
How many pixels are needed to match the resolution of the human eye? Each pixel must appear no larger than 0.3 arc-minute. Consider a 20 x 13.3-inch print viewed at 20 inches. The print subtends an angle of 53 x 35.3 degrees, thus requiring 53*60/.3 = 10600 x 35*60/.3 = 7000 pixels, for a total of ~74 megapixels to show detail at the limits of human visual acuity.
The 10600 pixels over 20 inches corresponds to 530 pixels per inch.
Aha! Not 326 ppi, but 530, at a distance of 20 inches! But who holds their iphone 20 inches away? I’d submit that many of us hold it only 10 inches away. This means that to achieve the highest resolution discernible by the human eye, the iPhone would need a resolution of 1060 ppi, or roughly 3200 X 2100 pixels! My 23-inch iMac has a resolution of just 1920 X 1200. So if you could shrink my computer display to the size of an iPhone display, you’d still need to triple its resolution to match the perceptual power of the human retina.
But there’s another problem with the “Retina Display” claim. The iPhone, along with all computer displays, can’t display nearly the range of colors that the human eye can perceive. Take a look at this diagram (source: Wikipedia):
This is a 2-D slice of the 3-D range of colors the retina can detect. The triangle represents the range of a typical computer display. See how much lies outside that display? That’s stuff you can see that your computer can’t show you.
So while the new iPhone display is indeed impressive, and leaps and bounds better than the old version, it’s still nowhere near the capabilities of the human retina.