Researchers in the U.K. have confirmed what many in the broadcast industry have known for years: most viewers have trouble detecting 8K—or even 4K—video resolution.
According to the researchers at the University of Cambridge and Meta Reality Labs, the human eye has a resolution limit: in other words, there are only so many pixels the eye can see. Above this limit, a screen is giving our eyes more information than they can detect.
To calculate the resolution limit, the researchers conducted a study that measured participants’ ability to detect specific features in color and greyscale images on a screen, whether looking at the images straight on or through their peripheral vision, and when the screen was close to them or further away.
The precise resolution limit depends on a number of variables, including the size of the screen, the darkness of the room, and the distance between the viewer and the screen. However, for an average-size U.K. living room, with 2.5 meters (~8 feet) between the TV and the sofa, a 44-inch 4K or 8K TV would not provide any additional benefit over a lower resolution Quad HD (QHD) TV of the same size.
The researchers have also developed a free online calculator where users can enter the size of their room and the dimensions and resolution of their TV to determine the most suitable screen for their home. Their results are reported in the journal Nature Communications.
Display resolution is considered equally important for the many other screens consumers use, on phones or computers, whether they’re used to take pictures, watch films or play video games, including games in virtual or augmented reality. Even car manufacturers are offering higher and higher resolutions for in-car information displays and satnav screens.
“As large engineering efforts go towards improving the resolution of mobile, AR and VR displays, it’s important to know the maximum resolution at which further improvements bring no noticeable benefit,” said first author Dr Maliha Ashraf from Cambridge’s Department of Computer Science and Technology. “But there have been no studies that actually measure what it is that the human eye can see, and what the limitations of its perception are.”
“If you have more pixels in your display, it's less efficient, it costs more and it requires more processing power to drive it,” said co-author Professor Rafał Mantiuk, also from Cambridge’s Department of Computer Science and Technology. “So we wanted to know the point at which it makes no sense to further improve the resolution of the display.”
The researchers created an experimental set-up with a sliding display that allowed them to measure exactly what the human eye can see when looking at patterns on a screen. Instead of measuring the specifications of a particular screen, they measured pixels per degree (PPD): a measurement of how many individual pixels can fit into a one-degree slice of your field of vision. Measuring PPD helps answer a more useful question than ‘how high is the resolution of this screen?’ Instead, it answers the question ‘how does this screen look from where I’m sitting?’
The widely accepted 20/20 vision standard, based on the Snellen chart that will be familiar to anyone who has ever had their vision checked, suggests that the human eye can resolve detail at 60 pixels per degree.
“This measurement has been widely accepted, 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,” said Ashraf.
Participants in the study looked at patterns with very fine gradations, in shades of grey and in colour, and were asked whether they were able to see the lines in the image. The screen was moved towards and away from the viewer to measure PPD at different distances. PPD was also measured for central and peripheral vision.
The researchers discovered that the eye’s resolution limit is higher than previously believed, but that there are important differences in resolution limits between color and black-and-white. For greyscale images viewed straight on, the average was 94 PPD. For red and green patterns, the number was 89 PPD, and for yellow and violet, it was 53 PPD.
“Our brain doesn’t actually have the capacity to sense details in color very well, which is why we saw a big drop-off for color images, especially when viewed in peripheral vision,” said Mantiuk. “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.”
The researchers modelled their results to calculate how the resolution limit varies across the population, which will help manufacturers make decisions that are relevant for the majority of the population: for example, designing a display which has retinal resolution for 95% of people rather than an average observer.
Based on this modelling, the researchers developed their online calculator, which enables people to test their own screens or help inform future buying decisions.
“Our results set the north star for display development, with implications for future imaging, rendering and video coding technologies,” said co-author Dr Alex Chapiro from Meta Reality Labs.
Over the past several decades as many television markets worldwide transitioned to higher definition video, BBC, NHK, EBU and ITU all conducted research that agreed with these findings.
