Retina displays - and, more recently, Retina HD displays - are often mentioned in discussions of Apple products. In this beginner's guide to Retina and Retina HD, we explain the definition of a Retina display, the difference between Retina and Retina HD displays, which iPads, iPhones, iPods, Macs and MacBooks have Retina or Retina HD displays, their pros and cons, the premium you are likely to pay for Retina screens where non-Retina options are available and whether (in our opinion) they are worth the extra money.

What's a Retina display? Or rather, what's the precise definition of a Retina display?

A Retina display is more of a marketing term than a precise technical term, but there is a definition: it refers to a screen on a computing device that has a high enough pixel density that the human eye can't make out individual pixels - or a general 'pixellation' effect - at all. In other words, the human eye is scientifically incapable of telling the difference between a photo of a painting shown on a Retina display, and the painting itself - in theory, anyway.

Retina displays are proprietary to Apple, by the way. So while a rival company could produce a screen to the same specs, it wouldn't be referred to with the same word. It's an Apple-trademarked term.

Retina display: iPad Air

My eye won't be fooled.

Perhaps not. But the scientific consensus generally backs up Apple's claims. It has been suggested that people with better than 20/20 vision might be able to pick out the pixels, but we've yet to hear from someone with that experience.

What resolution does a Retina display have?

That varies. Screen resolutions are given in the format '[number of pixels] x [number pf pixels', but the key factor in classifying a screen as Retina is pixel density, not the overall number of pixels. This makes sense, if you think about it: if you spread the same number of pixels across a larger screen, it will obviously be easier for the eye to pick out individual pixels. Pixel density is given as a single figure, measured in pixels per inch, or ppi.

And even in terms of pixel density there isn't a single figure that qualifies as Retina, since the equation also takes into account the distance of the screen from the eye. The required pixel density for each type of computing device - smartphone, tablet, laptop, desktop PC - is calculated based on a typical viewing distance. If you hold your iPad right up next to your face (you shouldn't do that, by the way) you may find that you can pick out pixels after all, because you are no longer using the device at the expected typical disance. Don't expect a refund.

The pixel density figures are: 326ppi (pixels per inch) for smartphones, since they are expected to be held the closest to the face; 264ppi for tablets; and 220ppi for laptops.

Are there better screens than Retina displays?

Yes there are. At any rate, there are screens out there offering a higher pixel density in the various categories listed above. Most obviously, since the iPhone 6 launch, Apple itself offers an upgraded class of screens that it calls 'Retina HD'. But many Android devices surpass not only the Retina screens but even the Retina HD models. The LG G3, for instance, has a flabbergasting pixel density of 534ppi, which blows even the iPhone 6 Plus out of the water.

Of course, it's worth pointing out that, according to our previous definition of Retina, it's debatable whether increasing the resolution/pixel density beyond Retina levels really produces a 'better' display. After all, what's the point in making a screen sharper if the human eye is no longer noticing any difference?

(Some would argue that, in moving up to a higher pixel density with the iPhone 6 Plus - as we shall see shortly - Apple made a tacit acknowledgement that a higher pixel density can be perceived by the human eye. Another possibility is that Apple simply didn't want rival device makers boasting about their superior screens, that no one can really tell the difference between 401ppi and 534ppi, and that we are all pretending the emperor is wearing clothes.)

Okay, what about a Retina HD display? What the difference between Retina and Retina HD?

The term Retina HD display - so far - refers only to the screens on the new iPhone 6-series handsets. Apple insists that both the iPhone 6 and the iPhone 6 Plus have Retina HD displays, even though they have different resolutions and different pixel densities from one another. The iPhone 6, indeed, has the same as the plain old Retina displays on the iPhone 5s et al, so we know that pixel density isn't the defining difference between Retina and Retina HD.

Retina HD display: iPhone 6

So what is the difference between Retina and Retina HD? Apple has been a little vague on this, but it appears to encompass four factors:

Higher resolution: A bit of a red herring, perhaps, since as we said before the iPhone 6 has the same pixel density as every iPhone since the iPhone 4, and pixel density is a more important metric than resolution. But yes, the iPhone 6 has a higher resolution than any previous Apple smartphone: 1334 × 750. This is mainly because it has a larger screen than any previous Apple smartphone.

The iPhone 6 Plus has a screen resolution of 1920 × 1080.

Higher contrast: I'm using Apple's own words here, but apparently the manufacturing method "involves using UV light to precisely position the display’s 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 next summer.

It's very likely that some or all of the next generation of iPad models will be rated as Retina HD - one or more of the iPad 6, iPad Pro and iPad mini 3, depending on what Apple unveils - although we will have to wait for the launch event to be sure.

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.

Non-Retina and Retina iPad display comparison

Non-Retina (left) and Retina iPad displays: a small but noticeable difference on text

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?

Generally Apple will make this clear - the Retina display is a big selling point, so it usually puts the word Retina right there in the product name. If you don't see Retina mentioned, it's probably not there. (The main exception is iPhones. These all have Retina displays and Apple has stopped making quite such a big deal about it - although it should still be prominently displayed in the tech specs section.)

But here are the product areas where Apple offers Retina displays, alongside any non-Retina alternative(s):

iPad

iPad mini: non-Retina. From £249 with 16GB of storage
iPad mini 2 with Retina display: Retina. From £319 with 16GB
iPad 4 (also known as simply 'iPad with Retina display'): Retina display. From £329 with 16GB
iPad Air: Retina. From £399 with 16GB

In other words, three out of four iPads have Retina displays. Going for the non-Retina iPad mini saves you £70 on the equivalent iPad mini 2 models, although you are also limited to fewer configurations - just the cellular or Wi-Fi model, both with 16GB of storage. The Retina model gets a range of storage options.

iPhone

There are no non-Retina options here: all iPhones currently available have at least Retina displays. The concept was introduced with the iPhone 4, and is also present on the iPhone 4s and iPhone 5iPhone 5s and iPhone 5c.

The iPhone 6 and iPhone 6 Plus both have Retina HD displays.

MacBook Pro laptops

13in MacBook Pro: non-Retina (from £899)

13in MacBook Pro with Retina display: Retina (from £999)
15in MacBook Pro with Retina display: Retina (from £1,599)

The non-Retina MacBook Pro is £100 cheaper than its approximate Retina equivalent, but in this case the comparison is difficult: the non-Retina model is much older and has numerous differences other than the displays. (It has a hard drive rather than flash storage, for instance.)

MacBook Air laptops

All of Apple's MacBook Air laptops are non-Retina. But there is a persistent rumour that a Retina MacBook Air will launch in the near future.

iMacs

Much like the MacBook Air range, Apple's iMac are currently non-Retina. Some believe a Retina iMac will launch this month; we shall see.

Are the Retina and Retina HD displays worth the extra money?

Let's start with plain old Retina.

For certain tasks, Retina screens are absolutely worth the money (although by this point you may well find that non-Retina alternatives are getting thin on the ground). If you're going to use your iPad mini for reading ebooks, for instance, you'll really benefit from a sharper screen. And if you edit photos on your laptop, they'll look a lot better in Retina form.

Bear in mind, too, that the price differences above don't just reflect the inclusion of a Retina display. The iPad mini 2 has a more powerful processor that the original iPad mini, for example, so it's much more future-proofed for handling apps and games in the future.

Check out the individual reviews for more information, and since eyesight isn entirely personal thing, we'd recommend going into an Apple store, or checking out a friend's Retina and non-Retina devices (side by side if possible) to see the difference for yourself. You may not even notice one, in which case your buying decision just got easier.

What are Retina and Retina HD displays, and are they worth the money?

What about Retina HD? That's harder to quantify, since the iPhone 6 and iPhone 6 Plus are such different propositions. They're lovely screens, that's for sure, but the main thing they offer over their non-Retina predecessors is size. The iPhone 6 Plus is super, super sharp, but the iPhone 6 has the same pixel density as the iPhone 5s.

We may feel differently after a few months with the devices, but our early feeling is that the 'Retina HD' criteria mentioned above are the least of the reasons why you'd be upgrading from your iPhone 5s - the polariser is quite nice for sunny days, the viewing angles seem to be very slightly better but will hardly affect your day-to-day experience, and the improved contrast hasn't blown us away. But then, the improved camera, bigger screen, redesigned chassis and prospect of Apple Pay will be what most potential upgraders are interested in, not the differences between Retina and Retina HD.