When Apple extended the iPhone's screen size from 3.5 inches to 4 inches with the iPhone 5, it increased the display resolution from 960 x 640 to 1136 x 640, packing 326 pixels per inch.
Now, code buried within the beta for iOS 8 hints that another upgrade is in store.
Developer James Thomson posted to Twitter that he noticed iOS 8 automatically chooses to load images at a triple retina resolution rather than a double retina resolution like iOS 7 does. 9to5Mac was the first to spot this tweet, and has also independently confirmed the same.
So what does this mean? It likely means that the iPhone 6 will come with a screen that's larger and sharper than the iPhone 5s.
Daring Fireball's John Gruber explains how this works. At a double retina resolution, each point in the iOS user interface would be represented by two physical pixels in each dimension. For instance, a 2 x 2 square would include four pixels. Therefore, a single point in a triple retina resolution would cram nine pixels into a 3 x 3 square, Gruber says.
You may be thinking- "Wait, Apple says the iPhone has a retina display. What do you mean by double retina resolution?"
It's important to note that these double and triple resolutions refer to how the user interface (i.e. iOS and the apps you use) are scaled to look on your phone's screen. As 9to5Mac previously explained, the iPhone 5, 5s, and 5c have a "base resolution" of 568 x 320 from a developer's perspective. However, these phones actually have twice as many pixels to display images at a 1136 x 640 resolution although developers are creating apps at a 568 x 320 scale. Since they have twice as many pixels as the base resolution, this is referred to double retina resolution.
Usually, the code in iOS would choose an image that fits the current device's screen scale, as 9to5Mac notes. For current iPhones, that would be a double retina resolution. But the latest iOS 8 beta continues to load a test image at triple retina resolution rather than double, according to 9to5Mac and Thomson.
Take a look at the test images below. The first is a test image at retina resolution, while the second shows double retina resolution, and the third depicts triple retina resolution. Notice how much sharper the third image is compared to the first two.
Of course, it's always possible that this is a bug within the beta for iOS 8. But Thomson doesn't think that's the case. The iOS 8 beta won't load other test images at higher resolutions, leading Thomson to believe that there is specific code there instructing the software to run images at a triple retina resolution.
This doesn't necessarily pinpoint a specific resolution for the iPhone 6, but it does suggest that Apple is planning to squeeze more pixels per inch on the iPhone's screen. If the iPhone 6 does have a larger display than the iPhone 5s, it would make sens for Apple to increase its resolution as well, just like it did for the iPhone 5 in 2012.
The company will presumably unveil its next iPhone at an event on Sept. 9, so we expect to learn more soon.