Retinal resolution of the human eye: how many pixels can we perceive on 4K and 8K TVs?

What is the retinal resolution of the human eye - in other words, roughly how many “pixels” can we really make out? And, for day-to-day watching at home, does it actually make sense to spend more on an ultra-high-definition television to get the best viewing experience?

Researchers at the University of Cambridge, working alongside Meta Reality Labs (who develop virtual and augmented reality technologies), recently examined those exact questions.

Retinal resolution, pixels and the typical UK TV viewing distance

Their results indicate that, from the usual sofa-to-screen distance in an average UK living room, our eyes cannot distinguish every pixel shown by ultra-high-def 4K or 8K televisions.

As a result, these higher-resolution panels appear to deliver no obvious advantage over a lower-resolution 2K television of the same size (44 inches).

How the researchers tested what viewers could perceive

To reach this judgement, the team assessed how well people could pick out particular visual features on a screen - for example, patterns with very subtle gradations. They varied the viewing conditions by using images in colour and in grey, changing how far participants sat from the television, and comparing straight-ahead viewing with what could be seen using peripheral vision.

If a participant could make out the lines within an image, that was taken as evidence that their visual system could resolve detail at that level.

The study involved 18 viewers, aged 13 to 46 years.

Pixels per degree (ppd) and the limits of eyesight

The researchers also looked beyond a simple notion of “resolution” by calculating pixels per degree (ppd) - a measure of how many separate pixels can fit into each degree of your field of view.

Previously, scientists had assumed people could perceive detail up to 60 ppd, drawing on the commonly used 20/20 vision benchmark from the Snellen wall chart - the familiar rows of letters that get smaller line by line.

But the Snellen chart is something of an ophthalmological dinosaur.

"This measurement has been widely accepted," explains University of Cambridge vision researcher Maliha Ashraf, "but no one had actually sat down and measured it for modern displays, rather than a wall chart of letters that was first developed in the 19th century."

In the new work, Ashraf and colleagues found that the upper limit of what the eye can resolve is higher than earlier estimates, though it depends strongly on colour. For grey imagery, the limit is 94 ppd; for green and red, it is 89 ppd. For yellow and violet, however, it drops sharply to 53 ppd.

What this means for 4K and 8K TVs - and why colour matters

Taken together, these results suggest television design may now be hitting diminishing returns, at least where resolution is concerned.

In terms of screen size, larger sets will likely remain appealing. Even so, the researchers hope their findings encourage manufacturers to create displays whose resolution matches the visual capabilities of a larger share of people - for instance, 95 percent - rather than relying on the previously assumed “average” observer.

Yet it is not only the eyes that determine what we see clearly; the brain sets constraints too. Human perception is collaborative, and ocular resolution depends on the eyes, the brain, and the way they work together.

"Our brain doesn't actually have the capacity to sense details in colour very well, which is why we saw a big drop-off [in ppd] for colour images, especially when viewed in peripheral vision," explains Rafał Mantiuk, a computer scientist at the University of Cambridge and senior author of the study.

"Our eyes are essentially sensors that aren't all that great, but our brain processes that data into what it thinks we should be seeing."

This underlines a broader point: the apparent limitations of our vision have evolved, endured and spread because they are sufficient, rather than because they are flawless.

If manufacturers want to capture attention and keep viewers looking, it may be wiser to build screens that better suit a wider range of people’s eyesight.

This research is published in Nature Communications.