Most people agree that HD is better than SD, but is 4K better than HD? And what size screen should I get? When it comes to the increasing resolution of displays, there are plenty of opinions as to what is worthwhile and what is hype. There are also a number of guidelines around that you could follow, but are they always accurate or even appropriate? Ultimately these things all link together, so today I’d like to try and give you a you a bit of a scientific approach and some examples that will hopefully fill in some blanks for you.

Where to start?

The biggest variable in the mix is – naturally – the human being. More specifically, the Visual Acuity (VA) of the human being in question. The definition of visual acuity (by Meriam-Webster) is “The relative ability of the visual organ to resolve detail,” and is measured with reference to angular resolution. For pixel based media, this is expressed in Pixels Per Degree (PPD).

People with very good eyesight may be able to resolve around 120 PPD, although the average for adults is around 80 PPD. Fun fact: 20/20 vision is actually only around 60PPD, in other words, below average!

Let’s look at some examples

  • To match 80PPD on a 65” display with 1920×1080 pixels (Full-HD), you would need a viewing distance of 3,37m (133”)
  • To match 80PPD on a 65” 4K display with 3840×2160 pixels (4K UHD), you would need a viewing distance of half that of the Full-HD distance, so 1,68m (66”)

Dropping below 80PPD by getting closer to the screen, or using a larger screen with the same resolution, the average eye will start to break fine details up into into visible pixels. Going above 80PPD by moving away from the screen or using a smaller screen with the same resolution, the eye will start to merge pixels together, meaning that the finest details will start to get lost.

If your eyesight happens to be better than average, you might find yourself being able to see the pixel grid. Is this a bad thing? Try it yourself with your PC or TV. How close do you need to get to start spotting pixels? Do you feel you can see more or less detail by getting even closer? Is it bothersome when you start seeing the pixel grid? The answers to these questions may differ a lot between individuals.

To be on the safe side, it is probably best to design for above average visual acuity. A good starting point is probably 100PPD. Back to the examples:

  • For the same 65” Full-HD screen as described above, that would put you at 4,3m distance (168”).
  • For 4k that would be exactly half again, so 2,15m (84)”.

Now to make it a bit more interesting, let’s compare those numbers with THX’s recommendation for viewing distance, which is based on a 36 degree viewing angle (edge to edge on screen that is). For the 65″ screen from our example, this angle is achieved at almost the same distance we calculated for 100PPD at 4K resolution. In fact, because the ratios are fixed, we can state that this will always be the case for 4K resolution content. In other words, following the THX 36 degree guideline will put you in the 100PPD sweet spot for 4K.

However, going back to the Full-HD 65″ screen, the PPD would drop all the way down to 55 if you follow THX. This just goes to show that different interpretations of what is important can have a significant effect on the resulting recommended viewing distance.

Let’s flip it around now and come at it from the viewing distance perspective. Most rooms are not designed around a display. Displays are usually added to a room, where aesthetics, budget and other practical factors more often than not determine what size screen is acceptable. 4m is not an uncommon viewing distance, so let’s use that here. Matching 100PPD and/or 36 degree viewing angle at 4m on a 4k screen would require a screen size of 120 inches! Yep, not gonna happen! 🙂

With Full-HD, that would be a far more realistic 60″ screen, but your viewing angle would be down to 18 degrees. This puts things in perspective quite quickly when it comes to the question of what screen size to pick and whether or not to blindly follow guidelines. It all depends on who’s asking and what the practical constraints are.

All right, enough examples, what can we conclude for all of this?

  1. Higher resolution does not automatically equal more detail, as the amount of detail you can actually see is determined by you visual acuity in combination with the viewing distance, and not simply by the resolution of an image. If the angular resolution already exceeds what you are capable of resolving, there’s no point adding pixels. Put another way, increasing resolution allows you to get closer to a display without the image starting to break into pixels, thereby allowing you to increase you viewing angle of the screen and altering your viewing experience. At this distance, you can see the same amount of detail as with the lower resolution screen from farther back, but now you are closer and therefore screen appears larger and there is more information to take in.
  2. People are different, situations will vary. There is no single correct answer. Sorry!
  3. Guidelines can be very useful, but using them out of context will make you look like an idiot, so be careful! 😀


Originally published on November 14, 2016by Edwin Edelenbos on LinkedIn.

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