What's a Retina display - and what's a Retina HD display? Do these terms actually mean anything?
Put simply, a Retina display is any display to which Apple has decided to apply the name. Retina is an Apple brand for which there’s no specific definition. There are however, features which set Retina displays apart from non-Retina screens.
The first, and most important is pixel density. When Steve Jobs launched the iPhone 4, and with it the first Retina display, he described it as having a screen with so many pixels packed closely together that they were imperceptible to the human eye at a distance of twelve inches. He went to great lengths to explain that, because the iPhone 4’s screen packed in 300 pixels per inch, most people wouldn’t see them at all when the phone was a foot from their eyes.
Since then, Apple has launched several devices with Retina displays. Some have pixel densities of more than 300 pixels per inch, some with less. How can they all be called Retina? Because there are two crucial elements to whether or not pixels are perceptible: density and distance. The further your eyes are from the screen, the lower the pixel density needed to make the pixels ‘disappear.’
Generally speaking, the bigger the screen, the further your eyes are likely to be from it and so the lower the pixel density required to ‘qualify’ as a Retina display.
So, for example, iPhones from 4 to 5s had a pixel density of 326 pixels per inch, while the current 27in iMac has only 218 pixels per inch. Given that, for most of us, the distance we sit from a 27in screen is more than one and a half times the distance at which we hold an an iPhone from our eyes, the lower pixel density is irrelevant.
How does a Retina display compare to non-Retina?
Non-Retina (left) and Retina iPad displays: a small but noticeable difference on text
In simple terms, Apple ‘converts’ a device’s display to Retina by doubling the number of pixels vertically and horizontal, meaning it has four times as many pixels as its non-Retina counterpart.
If it did that and nothing else, however, there would be a problem. User interface elements like menus and icons would look tiny. To compensate for this, Apple created what it calls HiDPI mode, where each interface element is doubled in size vertically and horizontally and so appears at the same size as it would on a non-Retina display.
The effect of a Retina display is to make everything look more crisp. Text, especially, benefits from Retina – it looks smoother, with the curves on characters looking like curves instead of jagged steps.
What’s Retina HD and Retina 4K/5K?
Steve Jobs painted Apple into something of a corner when he described the pixels in the iPhone 4 as imperceptible to the human eye. Where do you go from there? How do you describe a screen that’s even better? Apple’s answer has been to borrow terminology from the video and broadcast industries.
When the iPhone 6 came out in 2014, it had a vertical resolution of 750 pixels, a little more than the 720 pixels which forms one of the standards for HD video. The iPhone 6 Plus had a vertical resolution of 1080 pixels, exactly the number of pixels needed for the higher of the two HD video standards. Never one to miss an opportunity for a catchy label, Apple labelled the iPhone 6 and 6 Plus’ displays ‘Retina HD.’ It did the same in 2015 with the 6s and 6s Plus when they launched in 2015 with the same screen resolutions.
HD, however, is relatively old news in the world of video. Today, the highest quality TV screens have 4K displays, which have 4,000 pixels horizontally. So when Apple launched a 27in iMac with 5,120 horizontal pixels in 2014, it used the same naming convention and called it ‘Retina 5K.’ In October 2015, it launched a 21in iMac with a horizontal resolution of 4096 pixels (and a pixel density of 218ppi), and called its display ‘Retina 4K.’
Which Macs, iPads and iPhones have Retina displays?
As of the end of 2015, the only Apple device (iPod nano excepted) with a built-in display that doesn’t have Retina, at least as an option, is the MacBook Air. That may be rectified in 2016, or it may be that, following the launch of the MacBook earlier in 2015, the MacBook Air is phased out. There are also a 21.5in non-Retina iMac and the now-ancient MacBook Pro with SuperDrive (which hasn't been updated since 2012).
What about the Thunderbolt display?
Sadly, despite having launched a 27in iMac with a Retina display in 2014, Apple has yet to upgrade its Thunderbolt display to Retina. Its resolution is 2560 x 1440 pixels. To put that in context, it has the same number of pixels horizontally as a 13in MacBook Pro and horizontally as the 12in MacBook.
While Apple still sells the Thunderbolt display on its website, it’s failure to upgrade it in more than four years would seem to indicate that it has no enthusiasm for promoting it, making it unlikely it will be upgraded to Retina soon, if at all.
Do other manufacturers use Retina?
No other manufacturers use the name Retina – Apple’s lawyers would soon have something to say if they did. But several makers of smartphones and tablets, in particular, have displays with pixel densities which are at least a match for Apple’s devices, and in some cases far exceed it.
Examples include Sony’s Xperia Z5 Premium which has a resolution of 3,860 x 2,160 pixels – just shy of 4K – on a 5.5in screen, giving it a pixel density of a monstrous 806ppi. And Samsung’s Galaxy S6 has a resolution of 1440 x 2560 pixels – known as Quad HD or QHD — on a 5.1in screen, giving it a pixel density of 577ppi.
Whether there’s any point in having 4K and QHD resolutions on a smartphone is debatable. If Steve Jobs’ assertion was right and 300ppi on a smartphone is enough to make pixels invisible, then squeezing in more pixels would seem to be redundant.
A Retina display then is not one with the highest resolution available, but may be the optimal balance between image sharpness and the power needed to provide those images.