Why the iPhone 6 probably won't have a Full HD screen
Apple is bound to go for a higher resolution on the iPhone 6, as previous form demonstrates
One of the only things that everyone seems to agree on is that the iPhone 6 will have a larger screen with a higher resolution.
So far, the rumours suggest that it will have a 4.8in Full HD (1,920x1,080) display. In a way that makes a lot of sense, as it's the typical resolution we've seen on screens this size, such as on the Samsung Galaxy S4. However, Apple doesn't tend to do things because other companies have done them. We've seen that with the iPad Air and iPhone 5S, both of which have the same size screens and the same resolutions as their predecessors, even though they're no longer the highest-resolution tablet or phone.
What seems to be important with Apple is the user experience and keeping that consistent across all of its devices. It's from this information that it would seem that Apple wouldn't want to jump to a Full HD resolution for the iPhone 6. Here's why.
Ignoring the increase in screen size expected for the iPhone 6, for now, we'll look at resolutions on the same size screen. So, given the same screen size, increasing resolution gives you two choices: you can either fit more on the screen or you can keep the same amount on the screen, but increase the detail.
The first option tends to be what Microsoft does with Windows. So, as you increase resolution you find that you've got more desktop space, and can fit more windows on screen, with the disadvantage that icons get smaller. You can see this in the illustrative screenshots below, which show you the same screens. The first is taken at a resolution of 1,280x720, and the dialog box takes up most of the screen and there's less space on the taskbar. The second screenshot is taken at 1,920x1,080 and the dialog box takes up less space and there's more room on the taskbar.
This approach works to a certain point, but push the screen resolution further and icons become smaller and smaller and harder to read. It's more of a problem on a smaller screen, such as an iPhone. Apple tends to use the second approach, which is to use more screen resolution to make everything look sharper.
For this example, we'll look at the iPhone 3GS and iPhone 4, both of which had a 3.5in screen. The difference was in resolution, with the iPhone 3GS having a 480x320 screen and the iPhone 4 moving up to the Retina 960x640 screen. The shot below illustrates this, showing the iPhone 4 (right) compared to the iPhone 3GS (left) both at the same size.
Side-by-side, you can see that the iPhone 4's resolution is much sharper than the iPhone 3GS'. Importantly, you don't get anything extra on the screen by increasing the resolution, just icons and text become sharper.
This example is slightly exaggerated, as you're looking at the image on a bigger screen. We also had to do some resizing to make both appear the same size. We took the iPhone 3GS screen and resized it to iPhone 4 resolution (640x960) in order to demonstrates the point.
What's clever about Apple's system is how it handles the increase in resolution. The iPhone 4 has four-times the resolution as the iPhone 3GS, with double the horizontal and double the vertical resolution. It meant that any app written for the iPhone 3GS could simply be scaled for the iPhone 4, keeping the same aspect ratio and proportions. In this regard the user experience of an app on the iPhone 4 is the same as on the iPhone 3GS.
If Apple had used a different resolution for the iPhone 4, say 1,600x1066, the scaling would be much harder, as that resolution is 2.5 times the iPhone 3GS'. Apps would have to be completely re-written and Apple developers wouldn't know exactly how their apps would look on any iPhone.
With the iPhone 5S, Apple increased the screen size to 4in, but crucially changed the aspect ratio, opting for a 16:9 widescreen display. Scaling in the same way wasn't possible, but Apple was, again, clever about how it increased resolution, just adding pixels to the top and bottom of the screen, opting for a resolution of 1,136x640.
This gave Apple enough room to add one more line of icons. As you can see from the image below, the iPhone 4 screen (left) fits neatly into the middle of the iPhone 5's (right) screen. Importantly, both have the same pixel density (the number of pixels per inch), so that the images both look as sharp.
This design is clever, as on the iPhone 5 it can run iPhone 4 apps exactly as they were designed by adding black bars to the top and bottom of the image. It meant that even with a new device, developers were sure that their apps would appear as they had designed them. When developers started making iPhone 5 apps, they could use some clever design tricks, so that apps would work on both devices properly. For example, the weather app on the iPhone 5 has an extra bit of information on screen, which can't fit on the iPhone 4. Crucially, even though Apple had increased resolution and used a new screen size, it never affected the user experience.
We can't see Apple wanting to change this policy in the future, but moving to a larger screen means that resolution has to increase. If Apple doesn't change resolution, the pixel density would decrease, making text and images look softer. This would stop the screen being a Retina display, which Apple clearly won't want.
Moving to a 1,920x1,080 resolution would increase the pixel density, but it would also break the ability for apps to be scaled neatly. This resolution is 1.69 times the horizontal resolution and 1.69 the vertical resolution (3.38 times the total resolution), which doesn't scale nicely. Take an icon, say, 20 pixels wide and multiply it by 1.69 and you'd get a final size of 33.8 pixels, which couldn't be displayed, as only whole pixels can be used.
What would make more sense is that Apple would quadruple the resolution again, opting for double the horizontal resolution and double the vertical resolution. This would mean that the iPhone 6 would have a resolution of 2,272x1,280. This would maintain the same user experience, while maintaining the sharpness that we've come to expect.
Now, Apple has also said in the past that its Retina screens are where you can no longer see individual pixels at a normal viewing distance. Going for a resolution of 2,272x1,280 could mean that the Retina threshold is exceeded, technically making some of the extra pixels pointless. However, we don't think that Apple will mind in this case, as keeping the user experience the same is more important.
We've already seen that on the 7.6in iPad Mini with Retina Display, which has the same resolution as the 9.6in iPad Air (2,048x1,536). On the smaller screen there's no need to have as many pixels, but keeping the resolution the same on both tablets, means developers know that their apps will look the same no matter which device they're used on.
Of course, all of this is conjecture, but introducing a third resolution that doesn't scale well from its existing products just doesn't seem like something that Apple will do. Unfortunately, we'll have to wait until the product's released next year to be sure for certain.