I read many articles that claim quadrupling the number of pixels with 4K over 1080p is subtle or requires a very large screen to see the benefit. During the course of my work I am seeing a considerable amount of 4K/UHD source material and display product. It is obvious to me that doubling the resolution in the vertical and horizontal plane (4K) done properly is actually a big improvement when done well. This article addresses the benefits of additional pixels above 1080p along with sample photos, experimental proof and theoretical analysis that this is the case.
Let’s first look at a series of experiments done by NHK that compared a plaster bust, model ship and butterflies to a display. These results can be found in ITE Technical Report Vol. 35, No. 16. The summary of the results are shown in the chart above. NHK claimed the tests showed 310 pixels/degree are needed for an image to reach the limit for human resolution. If this is true we should be able to benefit from 4K and even higher resolutions without changing the viewing angles used for comfort reasons. At a THX recommended 36 degree viewing angle this corresponds to a 11K display to hit the 310 pixels/degree limit NHK observed. 1080p is only 53 pixels/degree and far short of what the tests indicate we can see at THX recommended viewing angels. My own observations of properly performing 4K displays using quality 4K sources are that 4K is almost as big an upgrade as 480p was to 1080p.
Assuming these tests are correct 4K is useful out to about a 12 degree viewing angle. This corresponds to 17 feet for a 50 inch diagonal and 21 feet for a 60 inch diagonal display. Far beyond distances many people suggest for 4K. 1080p with the same criteria limits to about 6 degree viewing angle doubling the distances shown for 4K.
The ARRI film scanner results above show the results for a 2K, 4K and 10K film scanners. These scans are magnified from a section of 35mm film. I have heard in my work with the film industry that 12K is what is needed to replicate the best 35mm film. It is interesting that the NHK data indicated that 11K is needed to hit the limit of human vision for these applications.
The image above is a segment of 35mm film showing the difference in an image for 2K versus 4K. I have also seen “show prints” in Los Angeles which are prints that were made from masters instead of several copies removed. A 35mm show print in pristine quality is an amazingly detailed and high contrast image that exceeds what I have seen with 4K to this point, but it is getting much closer. Unfortunately, this media is extinct with the loss of film in theaters.
How can so many people be so wrong about 4K? I think it boils down to two false assumptions. The first is that vision is limited to 20/20 and second is that the digital media can capture all of the resolution at the resolution of the imager. Both of these are false. Here is a quote from an expert on human vision. August Colenbrander, M.D. (Smith-Kettlewell Eye research Institute and California Pacific Medical Center) “…emphasizes that, contrary to popular belief, 20/20 is not actually normal or average, let alone perfect, acuity. Snellen, he says, established it is a reference standard. Normal acuity in healthy adults is one or two lines better. Average acuity in a population sample does not drop to the 20/20 level until age 60 or 70. This explains the existence of the two lines smaller than 20/20: 20/15 and 20/10.”
The chart above shows the results of human visual acuity (corrected if necessary) versus age for several different experiments. It is obvious from the results that 20/20 is not where most people can be with their vision. The data is not as simple as this chart shows as these are trends among scattered results, but it is clear that the average for many people is closer to 20/15. This data clearly challenges the assumption that 20/20 is the limit for the typical person. I know in my own case my vision was about 20/10 when I was younger and has degraded to 20/15 at the current time. I am still better than 20/20 at 55.
The other important thing to realize is that one of the primary benefits of 4K is the fact that it reduces the image artifacts and softening caused from digitally sampling the analog world. The dominant theory for this was proposed in 1928 by Nyquist. This theorem simplistically states that when you sample a system digitally you need to sample it at twice the frequency of the information you are interested in at a minimum. This limit is why CD’s are sampled at 44kHz to be able to to pass 20kHz which is the common limit for human hearing. 4K has the bandwidth to allow the camera or film scanner to transmit an image properly to get a 1080p resolution on your 4K display if everything is done properly. This digital sampling problem is the reason that an animated movie from Pixar looks so much sharper on Blu-Ray than the best camera image which must be filtered to meet Nyquist to avoid horrible artifacts. Many 1080p cameras do not conform to Nyquist and have insufficient filtering which results in many image artifacts. Computer animation is not subject to the analog sampling problem and does not need to have sampling rate artifacts at 1080p because the image is actually rendered at 1080p and not sampled from the analog world.
Taking these two pieces of information into account one can calculate the number of pixels per degree required for a person with 20/15 vision which would be considered relatively common. 20/15 vision corresponds to 0.75 arc minute of resolving power. 2 pixels are required at the resolution limit at a minimum to meet the Nyquist rule for digital sampling.
Pixels/Degree = (60 arc minutes/ degree)(2 pixels/0.75 arc minutes) = 160 pixels/degree
(based on 20/15 vision & Nyquist sampling requirement)
The NHK data shown above indicates that more than 160 pixels is a benefit, but 160 pixels is where the chart takes a definite turn in slope. This indicates a more subtle improvement in pixels/degree beyond 160 pixels/degree. If you want to estimate where 4K is worth it based on this limit halve the distances above. This means a 50″ display is useful for 4K to about 8.6 feet, 60″ to 10.2 feet and a 50″ 1080p display is useful to about 17.2 feet. There are many likely reasons that more than 160 pixels per degree were required in the NHK experiment to hit the limit for human vision. These include factors like peoples vision can be better than 20/15 and less than optimal filtering from digital sampling increased the number of pixels required to avoid digital sampling degradation of the image. I would use 160 pixels/degree as the maximum distance for 4K at this time.
I believe these calculations along with the samples and test results shown give strong proof that screen resolutions greater than 1080p can offer significant improvements in picture quality at distances found in many applications. Real world experience will depend on source quality, display quality and system setup. As we push higher resolution more careful attention to detail will need to be exercised to obtain all of the quality improvements available.
Link to display resolution calculator based on your vision, display size and distance to the screen https://www.homecinemaguru.com/display-resolution-calculator/.