Researchers aiming to discover the limit of maximum resolution for displays have discovered that the limit of retinal resolution is higher than originally believed.
The research, titled Resolution limit of the eye — how many pixels can we see? was published in Nature Communications. Researchers from the University of Cambridge created an experimental setup with a sliding display to allow for continuous control of the resolution to measure the maximum resolution at which further improvements bring no noticeable benefit, identifying that the limiting factory may not necessarily be attributed to the retina itself.
The researchers aimed to determine whether the ultimate resolution at which an image appears sharp to our eyes with no perceivable blur, bringing in a level of control that other studies may have lacked. The team used achromatic (black-white) and chromatic (red-green and yellow-violet) resolution limits for foveal vision, and at two eccentricities of 10 degrees and 20 degrees.
The results showed that the resolution limit is higher than previously believed, reaching 94 pixels per degree (ppd) for foveal achromatic vision, 89 ppd for red-green patterns, and 53 ppd for yellow-violet patterns. The researchers observed a much larger drop in the resolution limit for chromatic patterns than for achromatic patterns, producing quantitative benchmarks for display development and implications for future imaging, rendering, and video coding technologies.
The resolution limit of the eye is influenced both by optical and neural factors, interacting in a complex and, what the researchers described, as an “unintuitive” manner. When light enters the eye, it is scattered in the ocular media and imperfectly focused on the retina due to optical aberrations. This diffraction on the pupil restricts the maximum frequency that the eye can resolve, leading to a resolution limit that can vary with the viewing distance and resulting in a higher resolution limit at larger distances.
The researchers found that the widely accepted 20/20 vision standard, the historically accepted threshold for human visual resolution is outdated, with younger observers with no optical abnormalities usually featuring acuities higher than 20/20.
In the context of displays, the researchers highlighted that the 13-in Ultra Retina XDR display found in the seventh generation Apple IPad Pro has an effective resolution of 65 ppd when viewed from 35 cm away, the shortest comfortable viewing distance. Both the 20/20 assumption and the retinal display resolutions are significantly lower than the population mean of 94 ppd, with the reserachers identifying individual values as high as 120 ppd, demonstrating that the 60-65 ppd range is not the ‘retinal resolution’ for a display.
The researchers measured the detection threshold for text, demonstrating that both black-on-white and white-on-black (dark mode) yielded results that show that the resolution limit of the eye is higher than broadly assumed, casting doubt on the common practice of chroma sub-sampling found in lossy image and video formats.
The new data suggests that the assumption of chroma subsampling, the resolution of chromatic channels being reduced twofold in relation to the achromatic channel due to the lower sensitivity of the visual system to high-frequency chromatic contrast, only applies for the yellow-violet colour direction, with a maximum resolution of 53 ppd. The researchers suggest that this does not apply for the red-green direction, consistent with the theory that the isoluminant red-green pathway is the most sensitive opponent-colour channel of the human visual system.
For periphery, a rapid decline of the resolution limit was found primarily due to the fall-off in cone density and the increase in receptive field size in the retina.
The data shows that the resolution limit declines with increased eccentricity differently across colour directions, with the achromatic resolution limit declining 2.3 times while red-green declines 4.9 times and yellow-violet 4.8 times. The results suggest that popular techniques, such as foveated rendering or foveated compression are optimised for achromatic vision and could provide further computational and bandwidth savings by lowering the resolution requirements for the chromatic channels.
The researchers aimted to translate resolution limit, expressed in ppd units, into actual displays and viewing distances, plotting the relationship between the display resolution and the viewing distances, measured in display heights.
As full HD resolution was not distanced to deliver a perfect image, older models indicate that a distance of at least 3.2 display heights would fall short of the reproduction below the visibility threshold. Using the researchers’ model, they found that a distance of at least six display heights would be necessary to satisfy the acuity limits of 95% of observers.
For 4K and 8K displays, previous models suggest that viewing distances of 1.6-3.2 and 0.8-3.2 display heights respectively were ideal, however the new model shows that there is limited benefit of 8K resolutions when sited further than 1.3 display heights from the screen, providing a framework to update existing guidelines and to establish new recommendations based on the limitations of the researchers’ vision.
An online display resolution calculator is available to allow users to test their own display here.
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