Does the “Retina Display” live up to its name?

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.

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13 Responses to Does the “Retina Display” live up to its name?

  1. Josh Rosenau says:

    But my computer is displaying all the colors outside the triangle. Magic!

  2. dave says:

    That reminds me of old TV commercials (or new ones, for that matter) for television sets. How are you supposed to tell that the TV in the commercial is any better than the one you’re watching?

  3. Destry says:

    It’s just a name. And a good one. What’s the big deal? Geez.

  4. Sam says:

    I was pondering the resolution of the eye before i went to see avatar at the imax. I did some calcs and came up with a figure of around 100 mega pixel for both the eye and the Imax screen. Since the imax screen completley fills your field of view the imax is the only place you can experiance an image comparble to your everyday view of the world.

  5. Joshua says:

    Your making this big hoopla about the name of the display, and your citing Wikipedia?

    I believe Jobs said that the eye can see 300 ppi at 10 inches, that calc is clearly at 20, so you would have to recalc.

  6. dave says:

    Joshua,

    You should read more closely. I got the figure from Wikipedia, but the citation is a professional photographer who happens to have done a good review of the relevant scientific literature, as well as conducted his own experiments.

    And I do recalculate for a 10-inch viewing distance.

  7. Nate says:

    Hi Dave –

    I was also recently questioning this “Retina display” claim myself and came across your post and it was a good starting point to answering my questions. The example from the clarkvision site is correct, except that to assume .3 arc minutes as the maximum acuity of the human eye is pretty bold. That would correspond to reading about the 20/7 line on the chart when getting your eyes examined. 20/7 is theoretically possible based on the spacing of the photoreceptors, but we consider 20/20 “perfect” and acuity better than that is fairly uncommon. Using the example, and assuming “only” 20/20 vision of the user results in (53*60)/1 x (35*60)/1 = 3180 x 2100 or about 6.7 MP. For the 3180 pixels width along the 20 inch side, you get 159 ppi. Double that for holding the phone at 10 inches and you’re at 318 ppi, which is less than the 326 ppi of the iPhone4. I’m an Android and PC guy myself, so I really wanted Steve to be wrong on this one, but it looks like for most people he is actually correct.

  8. sam says:

    interesting article thankyou, may be of interest to people reading this. The highest resolution dysplay that i can find to buy is at kopin . com and given that the original is only 0.97″ and 1280×1024 a 4″ dysplay should get near 5278×4222. So we have the dysplay tech. Now need to wait for the rest of the phone to catch up.

  9. ROFL says:

    @ Nate,

    Hilarity always ensues when people in blog comments try to discredit authors that have done their homework. You should have done yours too. 20/20 is NOT perfect vision, it is just the minimum cut-off for good vision. Most people with glasses will have better than 20/20 and even people that have 20/20 vision without glasses can get glasses to make their vision better as well, though it’s not really as necessary. Therefore, they should definitely strive to make higher DPI screens, not just high enough DPI to only cater to the low end cut-off for good vision. I use glasses myself and have better than 20/20 vision with them and I would love to see or use a screen with high enough DPI for my vision.

  10. Just says:

    Ha ha. Well their is a simple answer, ask to look at your friends iPhone, do you see pixels or don’t you? I do cause I have young eyes. Older people like 30’s or more will have a harder time picking out the details. Of course that depends on genetics and how well you took care of your eyes.

  11. pee pee eye says:

    It’s made for Steve Jobs’ eyes, which were able to see only 300 ppi. ;)

  12. Engineer says:

    Those numbers are actually off. Whoever munched them out didn’t account for the fact that a flat object, such as a picture, doesn’t have the same distance to any point in space across it’s whole surface.

    Here let me show you how it’s done. To keep the same distance to a point, we’re gonna bend that picture, and since we can figure out the pixel density with just one dimension, we’re gonna bend it over a cylinder and slice it on the xy plane, leaving us with a 20″ arc with all of it’s length distributed 20″ away from it’s focal point, or the eye for our intents and purposes. A 20″ arc with a 20″ radius subtends an angle of 1 radian, which is aprox. 3438 arc-minutes. Divide that by the 0.3 arc-minute per pixel figure that he somehow came up with and you have 11460 pixels, which along the 20″ of our arc gives us a pixel density of 573, and at 10″ it would actually be 1146.

    So there you go. A simpler formula would be this: P= 11459/D, with P being the max ppi you’re looking for and D the closest distance to the screen in inches. So for example a tv screen at 10 feet away would need 95 ppi. Now consider that a 1080p 30″ screen would only have 73 ppi, and bigger ones have even less. Can you tell appart two pixels on a full HD 30 inch screen at 10 feet away? Try it!

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