Do you really need 300 PPI on a 3.5 inch phone?
Update June 9 2010 – Let me be very clear that I am not suggesting that there are no benefits to the new Apple iPhone 4 “Retina Display” in this post. Not only is it a superb display and a massive improvement over the old iPhone and most other phones on the market, I’ve made it clear that the iPhone 4 is overall a revolutionary device. So this is not a bash iPhone post by any measure. What I am saying is that compared to a larger OLED screen with the slightly lower resolution of 800×480, the iPhone 4′s display is not advantageous for people who hold their phone more than 13 inches away from their eyes. I am also not suggesting that no one holds their phone closer than 13 inches. What I mean is that I don’t see many people holding their phone within 13 inches, and that we cannot discount the size advantage and superior contrast ratios of OLED over IPS technology.
Aside from the dubious advertising on the iPhone 4′s “Retina Display” where Apple grossly inflated the quality of the display, I was very surprised to see Apple use such a high resolution on such a small display. Apple claims that 300 Pixels Per Inch (PPI) is the limit of the human eyes when viewing something approximately 11 inches away from the eyes, and that the iPhone’s 326 PPI beats competing Smartphones with only 252 PPI. But how advantageous is 326 PPI in real life when most people typically hold the phone at 13 inches away or more?
If we look at figure 1 below, I illustrate how viewing distance (distance between phone display and your eye) affects the maximum PPI detectible to the eyes. If a display with half the PPI is held at twice the distance, it will appear as though the pixels are the same size which results in the same angular pixel density measured in Pixel Per Degree (PPD). This is the same reason your finger can look as big as the moon when you hold it in front of the moon at the right distance from your eyes.
So if Apple claims 300 PPI is the limit at 11 inches (and the theoretical limit is 50 cycles per degree), then 252 PPI is the limit at 13 inches viewing distance. When I observe the majority of phone users, most people hold the phone more than 13 inches away from their eyes because it strains the arm and possibly the eyes when it is forced to focus at such a short distance. For my eyes (which are better than 20/20), 6 inches seems to be the minimal focal distance although it is a strained focus and requires me to cross my eyes.
So the numerous smart phones with 252 PPI 3.7″ (and soon larger) displays are actually not at a disadvantage in terms of pixel density beyond 13 inches viewing distance. Beyond 13 inches of clearance, the size of the display is far more important than the PPI. In fact, many users would consider size to be more important than PPI even if they can see the pixels. Furthermore, those 3.7″ displays based on OLED technology generally have better color and contrast ratios than the IPS display that Apple uses and it’s the reason OLED costs far more than IPS. OLED technology is so expensive that the 11″ Sony XEL-1 HDTV is still over $2000 which is twice the price of 40+ inch IPS HDTVs.
So while it’s possible that some consumers will benefit more from the iPhone 4′s 326 PPI display because they hold it within 13 inches of their eyes, prospective buyers should consider their own viewing habits and examine the tradeoff between size and the other benefits of an OLED screen. A simple way to measure is to take a regular 8.5×11 inch paper as a convenient 11″ ruler. This is not to say that the screen is a make or break proposition as there are far more other factors to consider, and I believe that the iPhone 4 the best overall phone as of June 2010. But consumers should understand what high PPI actually means and a more informed consumer is a happier one.
Update 6/10/2010 - Analyst Raymond Soneira came to a conclusion that the PPI resolution falls short of the human Retina.
“So, if you hold an iPhone at the typical 12 inches from your eyes that works out to 477 pixels per inch,” Soneira added. “At 8 inches it’s 716 ppi. You have to hold it out 18 inches before it falls to 318 ppi.”
These comments makes absolutely no sense. If he wasn’t misquoted (and I have to give Mark Hachman of PC Magazine the benefit of the doubt that Soneira was quoted correctly), then Soneira’s statement is completely opposite of what is true. PPI or Pixels Per Inch stays the same regardless of distance held to the eyes but what does change is that angular resolution increases as you move the display away from the eyes so you don’t need as much PPI for a larger and more distant display. This is why a 42″ 1080P HDTV only has 52 PPI and you can’t see any pixels on it because you’re sitting so far away. Soneira is suggesting that the iPhone 4 looks worse and worse as you move it further away which is a baffling. The correct conclusion is that the iPhone 4 overshot the PPI and not undershot.