The usefulness of resolution is a function of four factors:
- size of display;
- the original resolution of the image;
- distance from display; and
- visual acuity of the viewer
Simply put, any one of these factors limits the viewer’s ability to fully resolve an image. There are those who will claim to have perfect vision, in the same way that audiophiles claim to have “golden” ears, but the average person can resolve about 2K of resolution. Most films are now finished to digital intermediate, which is usually completed at 2K, regardless of the resolution of the original capture. Advances in technology and some older technologies [70mm, VistaVision, Technirama, IMAX] are able to produce resolutions greater than 4K, but resolution is only one factor.
I’ve mentioned size and distance, and these two interplay quite tightly. Simply put, the larger an image, the greater the resolution visible to the viewer, but it depends on how far you are from the image, since individual pixels are more visible at closer viewing distances. 2K usually resolves quite nicely at 1.5–2 screen widths from the image. The recommended viewing distance for 4K is 1–1.5 screen widths, as the pixels are necessarily smaller and more tightly packed.
The nice thing about 4K is that the standard is not entirely about the resolution. Indeed, a lot of media marketed as 4K UHD is derived from 2K intermediates, but 4K UHD adds other image improvements to the mix: HDR or high dynamic range, can increase the range of contrast between absolute black and purest white, but is something that needs to be applied judiciously, as older film stocks would have limited dynamic range by virtue of and the technology available at the time.
The second big thing in 4K UHD is wide colour gamut. If you’re familiar in any way with how computer display technology has progressed, displays and video cards were often touted by how many different colours they could reproduce. This is pretty much the same thing, but think about those millions of colours in terms of gradations between shades. It’s about accurate colour reproduction.
As for 8K and 16K, their utility to the average viewer is negligible. If a display can more accurately reproduce human vision, that’s where the technology should concentrate itself IMHO