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Pixel Density: Pixels Per Inch (PPI) Explained

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One of the key battlegrounds in mobile technology this year has been the race to produce smartphones and tablets with higher pixel densities. The latest smartphones have pixel densities of up to 468ppi. What does this mean and should pixel density be an important consideration when buying your next smartphone?

 

What Is Pixel Density and PPI?

 

The Eye.jpgA mobile phone display has thousands (or even millions) of individual pixels. Pixels stand for picture elements – they’re the basic building blocks on a smartphone display. Images are formed by directing the millions of pixels to change colour in unison. By changing each pixel to give the desired colour, smartphones can produce almost any image.

 

There are many factors to consider when comparing smartphone displays. A popular point of contention has been the way pixels produce light. Organic LED displays and LCD displays produce light in a different way. This leads to differences in how the displays look – OLEDs have a higher contrast ratio whereas LCDs give a brighter image. Other people will look at how individual pixels are formed – the subpixel arrangement can greatly affect the readability of a display. Finally, there’s pixel density. This has captured the most attention in the past year and it’s where we’ve seen the biggest developments.

 

Simply put, pixel density is a measure of how close together the pixels are packed on your smartphone’s display. The most popular way to measure pixel density is “pixels per inch”. This is often abbreviated as ppi.

 

The following diagram illustrates the meaning of “pixels per inch”. We’ve considered a hypothetical display that measures 1-inch in each direction. Pixels are shown as a grey square. Starting from the leftmost example, we have a display with just one pixel. The one pixel takes up the entire display. In this example, there’s one pixel for one inch of display. We say the display has a pixel density of 1ppi.

 

Moving rightwards to the next image, the number of pixels has been doubled in both directions. The pixel density has also doubled to 2ppi. The pattern continues as we move to 4ppi and 8ppi.

Meaning of PPI.png

As pixel density is increased, the display is able to show more details in an image. The following diagram shows the effect of increasing pixel density. We’ve shown a heart on our one-inch square display. As pixel density is increased to higher ppis, finer details can be seen. The image becomes progressively clear.

 

Example of PPI Doubling.jpg
An increased pixel density allows for more detail to be shown. Displays with a higher pixel density will give clearer and sharper images.

 

A key thing to note is that when pixel density is doubled, the amount of information shown is quadrupled. Taking an example from the diagram above, as we go from 1ppi to 2ppi the pixel density doubles. At the same time, the pixel count quadruples. From 2ppi to 4ppi, we see the same pattern. A doubling of pixel density leads to a quadrupling of pixel count. As we move towards smartphones with a higher pixel density, the amount of information shown increases even faster.

 

Pixel Density (Pixels Per Inch/PPI)

Pixels Per Square Inch

1ppi

1

2ppi

4 (double ppi = quadruple pixel count)

4ppi

16

8ppi

64

218ppi (Galaxy S II)

47,524

306ppi (Galaxy S III)

93,636

326ppi (iPhone 5 Retina Display)

106,276

441ppi (Galaxy S4)

194,481

468ppi (HTC One)

219,024

“Pixels per square inch” is a much better indicator of image quality. When pixel density doubles, the “pixels per square inch” will quadruple. Compared to the iPhone 5 (326ppi), the HTC One (468ppi) offers 43% more pixels per inch. However, the “pixels per square inch” has actually increased by 106%. Picture quality has improved by more than is expected.

 

How to Calculate Pixel Density

 

To calculate the pixel density of your smartphone, start by taking its resolution. You can look at the resolution in either the horizontal or vertical directions. Once you have the resolution, measure the length of the display. This should be recorded in inches (2.54cm = 1 inch). By dividing the two numbers, you’ll end up with the “pixels per inch” (ppi).

 

Here’s an example where we’ve calculated the pixel density for the Galaxy S4:

 

PPI Calculation.jpg

An example of how to calculate the pixel density on the Galaxy S4.

 

If you don’t have a ruler to hand, a quick search online will normally tell you the size of your smartphone’s display. Please note: this is usually given as a length along the diagonal. To give an example, the Galaxy S4 (pictured) has a display that measures 4.35x2.45 inches. Its size is usually stated as 4.99-inches: this is the length along the diagonal. You can use Pythagoras' theorem to convert this to a horizontal & vertical length – alternatively use an online calculator that can do it all for you.

 

Pixel Density: Today’s Smartphones & The Limits of Human Vision

 

iPhone Retina Display.jpgOver the past few years, smartphone manufacturers have worked on integrating displays with an ever-higher pixel density.

 

The battle for pixel density began in 2010 when Apple released the iPhone 4. The iPhone 4 had a Retina display with 326 pixels per inch. Under normal viewing conditions, Apple claims the Retina display is sharp enough that most people would be unable to distinguish individual pixels. With magazines typically printed at 300 dots per inch, Apple’s Retina display was comparable in quality.

 

In the years since, rival manufacturers have pushed the boundaries even further. Earlier this year, Samsung and Sony introduced flagship devices with a 441ppi display. HTC took things even further: the HTC One packs a massive 468 pixels per inch. Comparing the same area like-for-like, the HTC One packs twice as many pixels as the iPhone 5.

 

Research has suggested that at a typical reading distance of 1 foot (30cm), humans can see up to 720 pixels per inch. At the average viewing distance for a computer monitor (2.5 feet or 76cm), this drops to a maximum of 300 pixels per inch.

 

Handset

Pixel Density (ppi)

Newspaper Quality

200ppi

Samsung Galaxy Mega 6.3

233ppi

Samsung Galaxy S4 Mini

256ppi

Samsung Galaxy Note II

267ppi

Printed Magazine

300ppi

Viewing Limits of 20/20 Human Eye (2.5ft/76cm)

300ppi

Samsung Galaxy S III

306ppi

Apple iPhone 4S

326ppi (“Retina”)

Apple iPhone 5

326ppi (“Retina”)

BlackBerry Q10

330ppi

Nokia Lumia 925

334ppi

Samsung Galaxy S4

441ppi

Sony Xperia Z

441ppi

HTC One(2013)

468ppi

Viewing Limits of 20/20 Human Eye (1ft/30cm)

720ppi

The human eye can only resolve details up to a certain limit. Someone with 20/20 vision could resolve details up to 300ppi when their phone is held at a distance of 2.5ft (76cm). At a viewing distance of 1ft (30cm), this increases to 720ppi.

 

When choosing your next smartphone, it’s important to look at the pixel density on the display. A smartphone with low pixel density may give poor-quality images. Text legibility will also be limited – particularly on webpages where text is small. On smartphones with a higher pixel density, the legibility of text improves and further details can be seen in a photo or video. A high-end smartphone should have a pixel density of at least 300ppi.

 

Your Thoughts…

 

In this article, we’ve looked at pixel density on smartphone displays. Over the past few years, manufacturers have raced to develop smartphone displays with higher pixel densitys. The latest smartphones can have pixel densities of up to 468 pixels per inch. Compared to a printed magazine, the latest smartphones pack twice as much information into the same space.

 

Is pixel density an important consideration when buying a new smartphone? Can you tell the difference between a Retina display on Apple’s iPhone and the higher pixel density displays from HTC, Sony and Samsung? Do you think we’ll continue to see displays with an even higher pixel density? We’d love to hear your thoughts… please drop us a comment below and let us know what you think!

 

Ken Lo writes about mobile technology and the mobile industry at Ken's Tech Tips.

12 Comments
guide

Really interesting, thanks!

beginner

A much misunderstood issue very well explained.

Many thanks for putting this blog together.

It definitely helped me to understand the topic a little bit better.

Cheers.