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PPI, Retina displays, and what makes a screen ‘good’

July 17th, 2016

Retina PPIIf you’ve read any reviews recently about phones, tablets, and other gizmos, you’ve probably come across a lot of talk of PPI (Pixels Per Inch) and “Retina” screens.

In short, the number of pixels crammed into a device’s screen seems to be uber-important to which gizmos get the best reviews.

But hang on a minute… Just what do these numbers mean?

Does it really matter that much?

PPI stands for Pixels Per Inch, which simply means how many pixels are stuffed into a screen – per inch. This metric has become a major selling point of smartphones, tablets, and even computers. This simple fact should be your #1 indicator as to what’s really going on here: driving sales!

Apple was of course first to popularize the idea of a high-PPI “Retina” display. It didn’t end there, though: Many smartphones these days have FullHD – or even 4K – 5″ screens! That’s nuts.

In recent times, sales of smartphones and tablets have plunged off a cliff. Sales of puters have also naturally dropped since most people don’t have tons of spare money stuffed under their mattresses.

But getting back to the technical side of things, exactly what does a “High PPI” screen mean – practically speaking – for you, the end user?

To understand PPI, you have to take two factors into account:

  • Screen size – measured diagonally, corner-to-corner
  • Screen resolution – X pixels horizontally by Y pixels vertically

A 27″ iMac with a 5K display has a 27″ screen with 5120 x 2880 pixels. Using our handy online PPI calculator, we have a PPI of 218.

But hold the phone: the iPad Air 2 has a 9.7″ display and a resolution of 2048 x 1536, which is a PPI of 264

Or, you may have a 5″ Android smartphone with a FullHD resolution of 1920 x 1080. That’s 440 PPI!!

Does that mean that the iMac has a crappier screen than an iPad, and the iPad has a crappier screen than the Android phone (218 < 264 < 440)?

Not so fast…

As I mentioned, PPI is only a measure of the number of pixels crammed into a screen of a given size. This number says nothing about the various other very important factors that determine the practical visual quality of the images on a display:

  • Brightness level – usually measured in nits
  • Contrast ratio – 300:1, 500:1, 1000:1, etc.
  • Viewing angle – at what angle the screen image fades
  • Color reproduction – standardized measure usually given as a percentage of “full color reproduction” – the higher, the better
  • LCD vs IPS LCD vs AMOLED vs OLED – the display technology used

There is also a very personal aspect involved here: different people prefer different types of screens!

SmartphoneNow that you get the idea that “it’s complicated”, let’s get back to PPI.

Let’s say you’re shopping for a tablet. You might pick the iPad Air 2 because of it’s 264 PPI display.

You read another review of a budget tablet running Android with a 9.7″ screen that “only” has a resolution of 1024 x 768 pixels. That works out to 132 PPI.

Seems low, right? Must be a crappier screen!

But again, hold the phone… Let’s look at some other numbers:

  • 5″ Smartphone (1920 x 1080): 440 PPI
  • 15″ Standard Laptop screen (1366 x 768): 104 PPI
  • 22″ FullHD (1920 x 1080) computer monitor: 100 PPI
  • 55″ 4K UltraHD Television (3840 x 2160): 80 PPI
  • 55″ FullHD Television (1920 x 1080, which happens to be Blu-ray resolution): 40 PPI

You might have noticed something here: the smaller the screen is physically, and the higher the resolution, the higher the PPI will be.

But that doesn’t necessarily translate into a “good” image quality. Some gizmos have high-PPI displays, but the color reproduction is very poor – which is very important if you are, say, an artist!

Furthermore, when you watch a Blu-ray movie on your 55″ TV, it looks pretty good, yes? That’s only 80 PPI!! In fact, your 15″ laptop with “low-res” screen has a higher PPI than your fancy big-screen TV – even if it’s a 4K screen! You don’t really notice, do you?

And that’s the point.

PPI is a selling point. For most people, it’s nothing more than a marketing gimmick. Oh sure, more pixels generally means sharper images. But are you really gonna notice that your 5″ FullHD smartphone screen (440 PPI) looks soooo much sharper than your 55″ 4K TV (80 PPI)? Probably not. You’re probably going to be quite happy with your smartphone, and also quite happy with your 4K TV.

So, the next time you read a review for a device, and the reviewer says something like,

It’s a great little tablet, but the 9.7″ screen only has a resolution of 1024 x 768, yielding a measly 132 PPI. That just can’t compare to uber-awesome Retina displays.

…you will know that this “measly” 132 PPI is actually BETTER than:

  • Your 15″ Standard Lappy screen – 104 PPI
  • Your 22″ FullHD puter screen – 100 PPI
  • Your 55″ FullHD television – 40 PPI
  • And even your neighbor’s 55″ 4K television – 80 PPI

Does that sound like a “crappy display” to you?

And then when you stop worrying about PPI, you can pay more attention to things like the screen’s color reproduction, brightness, contrast, and viewing angles – all of which will make far more of a difference to your enjoyment of the gizmo’s screen!

One final note: Software these days plays a very big role in how text is displayed on your puter – it’s not just the hardware that counts. Ever heard of “font smoothing”? In Windows, you can run the ClearType font optimizer by clicking Start, and typing: ClearType

TA-DA! No more blocky fonts, and text in Windows will be adjusted to display perfectly for your eyeballs!

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  1. Jane
    November 1st, 2016 at 00:38 | #1

    I’m trying to decide if a particular monitor I fancy will lead to eye fatigue due to it’s PPI to screen size ratio & I’m just more confused with every article I read!

    The screen LOOKS good, but unless I sit in front of it for a whole day (buy it, in other words), I can’t get any info about it.

    So, this looks like a good place to venture this query. Would you say that there’s more or less chance of eye strain with the following monitor:

    http://www.philips.com.au/c-p/BDM3201FC_75/signage-solutions-led-backlit-lcd-display

    I do plan to sit at least 3ft from it.

    Cheers

    • November 1st, 2016 at 01:31 | #2

      3ft away from a 32″ 1920 x 1080 screen would mean much larger individual pixels, so it might look a little blurry (to me, anyway).
      Frankly, I’d find a similar model (size and resolution) at a local store and do a “test-view” just to get a general and quick idea of how it would look. But long term, it’s kind of hard to say. It depends on your eyeballs! 😉

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