When it comes to the subjective experience of sharpness, pixel density is the key factor, not resolution. The iPhone 6 has a higher resolution than any previous Apple smartphone - 1334 × 750 - but this is because it has a larger screen than any previous Apple smartphone. (Its pixel density is the same as every previous iPhone since the iPhone 4.)
The iPhone 6 Plus, whose screen is bigger still, has a giant resolution of 1920 × 1080. But in this case the pixel density is bigger too - the extra pixels, in effect, exceed the extra screen space in which they are squeezed - meaning Apple's phablet is sharper than any other Apple smartphone. This, we would argue, is the sort of quality that is implied by the term 'Retina HD', but the fact that the iPhone 6 qualifies too shows this not to be the case.
What else does Retina HD refer to? It's possible that Apple intends the term to encompass the other new developments seen in the iPhone 6-series handsets:
Higher contrast: I'm using Apple's own words here, but apparently the manufacturing method "involves using UV light to precisely position the displays liquid crystals so they lie exactly where they should. Better-aligned crystals deliver a superior viewing experience, with deeper blacks and sharper text."
Dual-domain pixels: Apple claims that these enable the iPhone 6 and 6 Plus to offer wider viewing angles. We're not sure; perhaps the iPhone 6 can be viewed when slightly flatter on to the eye than the 5-series, but it's hard to notice much of a difference.
Improved polariser: Pretty niche stuff, this. You can more easily view what's on the Retina HD display when wearing sunglasses. (We tested this out on Macworld's balcony and it's true that icons are indisuptably clearer and brighter when viewed through sunglasses on an iPhone 6 than on earlier devices.) Could come in handy in the summer.
How do Apple's non-Retina displays compare with its Retina displays?
Obviously this depends on the resolution, pixel density and so on of the non-Retina display, but it's a fairly safe bet that Apple will never sell a computing device with a fuzzy or unclear screen.
If you compare non-Retina and Retina iPad displays you can see there is a difference, but the non-Retina display is still good. If you hadn't tried a Retina display, you'd probably think it was great.
The main difference is noticeable on fine detail and text. But you will occasionally be able to pick up the pixellation effect - only slightly, but it is there.
Which Apple products have got Retina displays?
Here are the product areas where Apple offers Retina displays, alongside any non-Retina alternative(s):
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