How close do you need to be to a screen to benefit from an ultra-high-definition display? It is sometimes suggested that the difference will only be discerned on relatively large screens viewed from an uncomfortably close distance. That seems to be the implication of a recent BBC study, which suggests that only 1.5% of viewers would benefit from increasing resolution beyond high-definition. Yet the way we perceive images is rather more complex than this suggests. We do not expect them to get less clear the closer we get.

At the annual summit of the Digital TV Group in London, Katy Noland of BBC Research and Development presented a paper based on a survey of television viewing conditions in the United Kingdom.

The survey found that the median television viewing distance was 2.63 metres, which is about the same is it was ten years ago. However, the average screen size has increased.

That may not be surprising, as most rooms have not got any larger while large flat high-definition screens have become much less expensive.

In fact the housing stock in any particular country may be the main factor in how far away people can actually sit from their television screen.

The study reported that the median relative viewing distance as around 5.5 times the screen height. It is suggested that only 10% of people currently watching from a distance of 3 times screen height or closer could potentially benefit from resolution beyond that of high-definition television.

It should be noted that the research was based on a self-reporting online survey posted on the BBC web site that produced just over 2,633 responses, of which only 2,185 reported having a television in their home. The previous paper from BBC R&D in 2004 was based on a survey of just over 100 people.

The researchers are clearly aware of the limitations of the recent survey, which is based on self-selection users and self-reported data, with a significant skew towards younger, male viewers, biased towards London and the South East.

Respondents were asked the height and width of their television screen to the nearest centimetre and the distance they normally view to the nearest ten centimetres. They were also asked what size screen they would ideally have in their current home if money were no object.

The results suggest a reported median screen size equivalent to diagonal measurement of 36″ – 40″. When asked what size screen they would ideally like the median response was equivalent to 48″, with a small peak at the largest suggested value, which was 110″.

It was evident that people estimated viewing distances to the nearest half metre, so the researchers added random values of plus or minus 25 centimetres to their responses. The median still came out at about 2.6 metres, although some claimed to view from less than 50cm from the screen, while some reported watching from over 5 metres.

This is reportedly the largest survey of its kind that has been conducted in the United Kingdom. That suggests that even the BBC, with its annual licence fee budget of £3.7 billion, has very little idea about how its programmes are actually watched in the home.

One might imagine that a rather more rigorous survey of the actual, rather than self-reported, viewing environment, might be beneficial to an understanding of what people actually expect from their television experience.

It seems from the survey data there is very little correlation between reported screen size and viewing distance. We might assume that most people buy a screen that they feel suits their budget and home then view it from a distance that is generally dictated by the size of the room and the arrangement of furniture.

So why is viewing distance relative to screen size important? The designed viewing distance for high-definition television is apparently just over three times the screen height. This is based on the assumption that the pitch of one pixel is then at the limit of spatial resolution for the eyesight of the average viewer.

This metric is based on ITU Recommendation BT.2022 on “General viewing conditions for subjective assessment of quality of SDTV and HDTV television pictures on flat panel displays”.

The optimal ‘design viewing distance’ is apparently ‘the distance at which two adjacent pixels subtend an angle of 1 arc-min at the viewer’s eye’.

For a 1920×1080 high-definition image this is equivalent to 3.2 times picture height, while for a 3840×2160 ultra-high-definition image this is put at 1.6 times picture height.

This engineering wisdom has been widely interpreted as suggesting that people would need much larger screens, or to sit much closer to them, in order to benefit from the increased resolution of ultra-high-definition.

By extension, we often hear that there is no need for ultra-high-definition on screens of less than, say, 60″. Conversely, we even hear that people will need to view screens from closer distances to appreciate them.

The BBC study concludes that only 1.5% of respondents view from a distance of 1.5 times the picture height and would therefore benefit from ultra-high-definition.

Unfortunately, this is somewhat misleading.

Many people will tend to buy the largest screen with the highest resolution that they can afford, justify, and fit through the door and in the room. Increasingly that will be a 4K or UHD capable display. They will watch it from wherever they feel comfortable, irrespective of any designed viewing distance. They will generally not be able to see the individual pixels that make up the picture from this distance. The point is that with an ultra-high-definition image will still not be able to see the individual pixels when they move much closer.

You see, people tend to move around their homes. They should not expect the picture on their television to get worse the closer they are to it. If anything, they might expect it to get better.

That is our general experience of the world. When we move closer to something we see more detail. When we move further away we assume the detail still exists.

We know that there are leaves on trees in the distance, even if we are unable to discriminate their individual forms.

There are no pixels in reality. So how far back should we stand to appreciate the real world?

It was refreshing to hear at in the same conference session from Ron Martin, who is responsible for innovation at Panasonic Hollywood Labs. He was much clearer of the need for increased resolution to improve image fidelity to a closer representation of reality.

He pointed out that the aim is not simply to achieve a resolution equivalent to an imaginary sensor in the human head. Human visual perception is much more complex than that. It actually operates by building up detail through constantly scanning and creating a mental model of the scene.

The concept of designing a system such that it is just good enough for the optical resolution of the average user is clearly based on engineering efficiency but may be insufficient.

Most people were satisfied with standard definition television when that was as good as it got. With the advent of high-definition it is apparent that a standard definition image is simply not good enough, especially when viewed at a relatively small distance on a larger screen. It breaks the illusion of the moving image.

So it is arguably desirable not to be able to see the pixels at any practical viewing distance, even on close inspection. That is one of the benefits of increased display resolution, even if it is synthesised through interpolation or ‘upscaling’.

The practical determinants of screen size are also related to the genre and composition of the image. A movie intended for theatrical release may benefit from a being viewed on a very large screen. A newsreader that appears much larger than life size might appear rather overbearing and unnatural.

Consequently, irrespective of practical or financial considerations, there is screen size that feels ‘about right’ in the average home. Over the decades this has increased in line with the technical quality of the image.

Then there are smaller screens that are typically viewed at much closer distances.

We are seeing an increase in viewing on handheld screens with a much higher pixel density that the average living room television.

An Apple iPad mini with a retina display has a resolution of 2048 x 1536 pixels at a pixel density of 326 pixels per inch. This higher than high-definition display is just 122mm high in landscape orientation. By ITU standards the optimal viewing distance is 39cm, which is roughly the comfortable distance when held by a bent arm.

Yet we can expect pixel densities for smaller screens to rise even higher, to reach or even exceed ultra-high-definition or 4K displays.

Bear in mind that a 300 dpi image on an A4 page has a resolution of 2840×3508 pixels, so a double page spread is about 20 megapixels. However, text may be imaged at a far higher resolution of 1200-4800 dpi.

Even an 8K 7680 x 4320 resolution 33 megapixel image would only be 65cm across when printed at 300 dpi. That is equivalent to a 30″ diagonal display.

So the answer to what is an ideal screen size, viewing distance or image resolution is not absolute. It depends on context.

If it is acceptable not to be able to see the pixels from the other side of the room then high-definition may be good enough.

But is it acceptable that the image should be less clear the closer you get?

If you do not want to see the pixels at closer viewing distances the optimum resolution may be much higher than some have suggested.

That does not mean that you have to sit uncomfortably close to a large screen in order to appreciate it.

A Survey of UK Television Viewing Conditions is available as a white paper from the BBC web site. Recommendation ITU-R BT.2022 is available from the ITU web site.