There's been a lot of speculation about the iPhone 5, in particular relation to the display. Will it be bumped up to 4 inches, or will it be bigger? Or, will it stay at the current 3.5 inches that people know and seem to love?
There are two problems facing Apple with regards to increasing the size of the screen. The first is that increasing the size of the screen without altering the resolution would cause the pixel-per-inch ("ppi") count to drop below that of the current iPhone 4S.
Another problem is that small tweaks to the screen resolution or aspect ratio could break the way current apps are displayed, requiring developers to rewrite their apps to support the new screen.
Take one example. Scaling up the current iPhone 4S 960×640 Retina display screen from 3.5-inch to 4.65-inch while keeping the same resolution would mean that the pixels per inch number would fall from 326ppi for the iPhone 4S to something in the region of 250ppi for a 4.65-inch screen.
That would represent a huge drop in pixel density. It's unlikely that Apple would bump up the screen resolution to accommodate for the larger screen because this would introduce a whole host of scaling problems for existing apps, essentially drawing a line underneath backward compatibility with existing apps.
Doubling the resolution of the iPhone's screen to 1920×1280 would make scaling simpler. It's what Apple did when it went up to the Retina display on both the iPhone and iPad. But such a screen would have a pixel density in the region of 500ppi.
Believe it or not, it is entirely feasible to produce a screen of such density.
LG announced the world's first 5-inch fully-HD LCD panel for smartphones, the highest resolution mobile panel to date. The panel based on Advanced High Performance In-Plane Switching (AH-IPS) technology and features 440ppi at a screen resolution of 1920x1080, allowing full HDTV quality on a smartphone.
It's not the only panel to feature such a high pixel density. Toshiba has a 6.1-inch display running at 2,560x1600 offers a pixel density of 498ppi.
Both these panels are "Retina display killer" displays in that their pixel density is far in excess of the 326ppi panel used on the current iPhone 4S. Visually, 440ppi or 498ppi is indistinguishable from 326ppi -- because the pixels are too small for the human eye to discern -- but it would give Apple enough wiggle room to increase the size of the iPhone's screen, while retaining the same aspect ratio of 3:2 and keeping the screen compatible with current apps.
If -- and that's a big "if" -- Apple decides to change the size of the iPhone's screen, my guess is that we'll see a doubling of the screen resolution just as we saw with the transition to the iPhone 4 and the iPad 3.