It makes sense for Apple to double the pixel density of the iPad mini in order to bump the screen up to "Retina" display quality, but going up to 400 pixels-per-inch (ppi) makes no technical sense at all.
The current generation iPad mini features a 7.9-inch 1,024×768 display with a pixel density of 163ppi. This falls below Apple's standard for a Retina display screen, where the human eye is unable to distinguish individual pixels at normal reading distances for the device. The pixel density is well below that of the current generation iPad and iPhone, which have a pixel density of 264 and 326ppi respectively.
The way that Apple transitioned the iPhone and iPad to a high-pixel-density Retina display was by doubling the pixel density. This meant that handling the transition was made easier because non-Retina-enabled apps could be scaled up easily to the new resolution as the aspect ratio remained the same.
So it makes sense that when Apple transitions the iPad mini to a Retina display, it will double the screen resolution from 1,024x768 to 2,048x1,536, while keeping the screen size the same. This will bump the pixel density from 163ppi to 326ppi.
This keeps the iPad mini consistent with the iPad, and means that apps continue to be interchangeable.
But today, Patently Apple is reporting that Korean sources are saying that the iPad mini 3 will have a screen with a pixel density in excess of 400ppi.
This makes no sense for a number of reasons.
First and foremost, what possible reason could going beyond Retina display offer? Unless you have bionic eyes with microscope mode, once the pixels get so small that you can't see them, making them smaller offers no benefit, beyond perhaps giving marketing a number to run with
- Apple has doubled the pixel density of devices to give its devices a Retina display. This made sure that older non-Retina apps remained compatible. Bumping the iPad mini up to 326ppi makes sense, but going higher doesn't. Not only is it adding pixels for no reason, it would mean that existing apps wouldn't scale up properly
Why would Apple scale up the iPad mini's display twice? If the iPad mini 2 is going to go up to Retina display levels, why would Apple boost the pixel density again?
A screen of this density wouldn't come cheap. Why would Apple spend unnecessarily on a component that offers no end-user benefit?
Bottom line, it's highly unlikely that we will see an iPad mini with a 400+ppi display.