Tuesday, 25 November 2014

Optimal screen resolution on mobile devices

[Editorial] human physics facing technical limits

This time I take a more general view into the display resolution issues, including some facts about physics, some THX recommendations, and some hardware performance issues, in a hunt for an optimal resolution to any mobile phones and tablets.

Optimal screen resolution on mobile devices

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Your eyes matter

First, let's have your eye. If you haven't visited an optic lately, pick a visual acuity test here to get the best out of this article, telling you the maximum resolution in mobile devices for your full visual experience. Tweaks for the test:

  • Use ctrl- in your browser to zoom out, before you measure the line
  • If you think you have eyes of a hawk, enter only half of the distance you actually walk away from your display, and divide the latter number of your result by 2 (e.g. 20/20 letters are still visible for you from the double distance means you have 20/10 visual acuity)
  • Max your monitor contrast and reduce the ambiance light in your room
  • If you have a cordless mouse, point it to "Display" and take it with you.
  • Even better if you have a friend to check your letters, so that you don't need to walk there and back all the time
Now, for the ultimate viewing experience in a mobile device screen, we should try to reach your limits - what does it require? 720p? FullHD? 4K? Let's find out! But just before that, a short theory splash:

Visual acuity

Human hairs are 17-181 µm wide. Thin hairs are quite impossible to separate by eye, and any pair of hairs, even in different color, can't be separated from further than 150cm distance unless you happen to know there's more than one. Hawk reaches a visual acuity of 20/2, humans, in the ultimate best, 20/8 (I'm happy of my "hawk eyes" with 20/15). Nominal medical value is set to 20/20, meaning that you could separate 1.75mm objects from a 6m distance.

Normally you might use your mobile device anywhere between 30-60 cm from your eyes. Before going into the pixel level, let's see the view:

Mathematics, all devices

Arc for one hair in your view field is the smallest reasonable arc for 1 pixel in screen resolution which you're able to separate. For example, 0.1mm hair at 200mm distance has arc of 0.03 degrees.
  • Viewing angle of a movie screen, by THX recommendation, is 40 degrees
  • Having 8 inch tablet on your laps, using it landscape, fills about 30 degrees
  • Watching TV in your living room (distance 2.5 times width) fills 20 degrees
  • 4,5" mobile phone, portrait, uses 10 degrees of the width of your visual field
For a maximum visual experience of sharpness, we want to fill these views with hairs as thin as you are able to separate, right?

Resolution of a tablet device

Later we bring any device as close as possible, but let's select the upcoming Jolla Tablet, 7.85 inches, as a reference first. As close as 20cm from our eyes it fills our landscape by 40 degrees (note, same as sitting in an optimal seat in a THX movie theater)

What we can see, and in this case also want to see, are pixels as close as 0.03 degrees from each other. Easy math:
  • 40 / 0.03 = 1333 pixels in width
  • In 16:9 ratio, that's a bit more than 720p resolution
That's all we need to watch a movie from 7.85 inch screen, following the THX recommendations. Having any more pixels would be an overkill to our physics. Or would it?

Yep, you're right - there is a reason for higher resolutions as well. This takes place at the moment you take the device (or the hairs) a bit further - isn't that odd? Those hairs seems smaller when watching from further, but if your visual is any better than the nominal 20/20, you can still see them separated in 50cm distance - maybe even without your glasses. Go ahead and test! 

In 60cm distance those hairs have only 0.01 degrees difference in your view, and maybe you can still see them both. This is about the limit for human eye. The math, turned into pixels:
  • Tablet of 7.85 inches, in landscape, on your laps, has 20 degrees view angle
  • 20 / 0.01 = 2000 pixels
  • FullHD has 1920 x 1080 pixels. Great!
Jolla has selected QXGA (2048 x 1536 pixels) to their upcoming tablet, which is quite optimal to separate a hair from another. But some screens have even more, why? Well, more is needed only if you want (and you're able) to see thinner objects than hair - and only few people are:

The distance from where you can still see all the pixels of a display

If you took the test in the beginning, or you've visited an optic lately, you might know your visual acuity. Above, the visual acuity of human is close to maximum at 20/10 - find the resolutions you're able to experience at full for each display size and the desired viewing distance.

Below, the same for "normal" humans, reaching 20/20 value in visual acuity testing. The difference is remarkable:

Resolution of Mobile phones

1080p resolution, nowadays quite common in mobile devices, seems to reach its optimal performance for 5 inch display at 40cm  - but you can experience this level of sharpness only if you have 20/10 visual acuity.

Have the nominal value of 20/20, and your physics limits your visual experience to a 540p device with the same 5 inch display, 40 cm distance.

In practice, mobile phones are used at the same distances as tablets, so the desired resolution can be easily calculated right from the display size and the desired (minimum) viewing distance.

With nominal medical value of 20/20, Jolla phone's qHD display (540 x 960 pixels) can be viewed from a distance of 35cm without visible pixelation. If your eyes are better, more resolution would add more details (e.g. even thinner hairs) into your visual experience. But the resolution isn't a priority:

Color reproduction and build quality

Actually, more important issue than the resolution, especially on the IPS (LCD) displays, is the colour reproduction. We don't want one pixel to have color, and this color spreading to the pixels next to it because of bad build quality, softening the image in undesired way.

In some cheaper screens the numbers might be high, but the actual pixels mix up into a mess. This reduces the experienced visuals into a half, or even into a quarter, of what we expect looking at the specs.

Also ISF states that resolution isn't what should be looked at as the primary numbers. According to them, the most important aspects of picture quality are (in order): 1) contrast ratio, 2) color saturation, 3) color accuracy, 4) resolution.

Camera's megapixels - pure marketing?

When using a screen to watch a photo, all we need from a camera are the amount of pixels which the screen is able to show. 1920 x 1080 pixels (fullHD screen) has 1920x1080 pixels = 2073600 pixels ≈ 2.1 megapixels. Yes, that's all.

However, when you have more in your camera, you're able to watch the photos sharp using better screens. 4K screen is already able to show 8.85 megapixel photos in full sharpness. Have even more, and you're able to have quality prints of your photos:

High quality printers, equipped with high quality photo paper, are able to push 300 ppi into the print. Note: Please refer back above, and think if you're able to see these separated without a magnifying glass - but yes, 41 Mp camera can offer data to 65 cm wide printed photo in full sharpness - but again - depending on the build quality: It's not easy to push those pixels separated from each other into a small image sensor used in mobile phones, like e.g. Nokia PureView technology is trying to achieve. The quality of optics is another, remarkable issue in these levels of sharpness.

As an example of what I'm saying above, I've taken sharper images with 2 Mp Canon EOS D2000, compared to an 8 Mp semi-professional camera from later years. And I'm pretty sure this old beast would wipe the floors with a 41 Mp Nokia 808 PureView mobile phone camera as well - not in the image size, but watching photos side by side in two Full HD screens. And, due to the larger lens, also in low light conditions.


It's not about the pixels - don't go into marketing talks, be unlike and look beyond. Knowing your visual, question the build quality with the seller, and use your own eyes with colorful quality photos. Don't go for resolutions over your own eyes, and keep in mind that every pixel must be drawn too.

By "must be drawn" I mean that also the other hardware parts selected for the device should follow the display's requirements. Better resolution = more drawing = faster GPU processing, bigger battery, maybe even a better CPU needed. Sometimes, you get more having less.

If you got really interested, maybe start with calibrating your screen. Unfortunately, mobile devices offers only the brigthness setting, missing color, contrast and even gamma settings which better monitors have. Well, I actually added that link as it's great in comparing displays of different mobile devices. Open that with your tablet as well (useful for all devices with enough of resolution)

How did your device perform? Leave a comment! Example image:

(with Jolla phone, tap twice to fully enlarge and turn your phone landscape)

By the way

Charts above are for mobile devices, but you can use them for larger screens as well - just multiply both the display size and viewing distance by 10, reaching to 30"-100" televisions viewed from 2m-7m distance.

For example, if you have 50 inch Full HD television in your home, check if you're watching it from 2m distance. Sitting 1m further at 3m, you can experience the same with 720p television (with nominal visual acuity). Don't pay just for the pixels.

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Test image courtesy: lagom.nl
Published: November 25, 2014 21:39 UTC
Updated: November 27, 2014 02:02 UTC


  1. The screen of my PC shows 6 types of doted/lined squares and a grey circle of the midle with a line wich goes from black to white, with the degradation focused on the center of the line. Don't know what you expect us to say about what we see...

    I have an HP ZR24w 24" IPS screen. I adjusted it after buting with the lagom tests :) So maybe I'm not the typical user XDDD

    Good article ;)

    1. Thanks :) You should visit the test link for full information about both the example image (this one should be watched almost eyes closed, and an optimal result is pure grey area). This and other test images are fully explained in the testing page - I linked it as I found it very useful for a serie of displays, including mobile displays.

    2. As said I adjusted my PC screen with lagom tests just after buying it. I see all in grey in that test except the line of the center.

      In my Jolla I see the 5 of 6 examples, sothe sharpeness is too low. There is no way to adjust it, is it?

    3. On Sailfish Browser, tap the image twice to enlarge it full, and turn the phone to landscape for correct visuals. What I see is a circle in pure grey, which means that gamma should be adjusted.

  2. I see the 5th of th 6 examples, so in mine the sharpness is too low. The problem is that there isn't a way to adjust Jolla screen, is it?

    1. The test doesn't fully suit on Jolla, as it's old and not tuned up for a qHD displays. The "small" part has a few working images on the site. And no, sharpness of Jolla display can't be tweaked without going very deep into the code (which might not be even open, as it's a Qualcomm chip driver)