sheldoncooper
Mostly yes, I would say 5k is the minimum where scenes can be considered looking good, 4k is watchable, everything below basically unwatchable.
But the number of pixels is just one thing that determines quality, there are many other factors, especially the lens and also the post processing, and you can make a video that looks terrible in 5k+ too.
But generally I would say the jump from 4k to 5k+ is the biggest jump ever, most likely also to other factors then pure resolutions. Up to 4k many scenes where basically shot with something similar to GoPros or sometimes actually GoProsl But these are no better than a phone camera with a fisheye lens in front of it. With the transition to 5k+ there usually also was a transition to more serious cameras.
While 5k is nowhere near to be called enough, the 4k to 5k+ transition was probably the biggest we have ever seen and will stay that forever at least for porn.
In VR autofocus would be quite distracting (and actually is often even not possible with current lenses), and actually you do not even know at the time of shooting, because ultimately the viewer decides what to focus on. So the goto thing would be shooting with hyperfocal distance. But larger and therefore higher quality sensors, larger and therefore more light sensitive apertures and higher resolutions all reduce effectively DOF, and therefore hyperfocal distance gets farther away from the camera, too far for typical porn distances. So you have to find some compromise, to still get the front decently in focus, while at the same time keeping the background decently sharp.
It can already be seen in todays 7k and 8k videos that outside the sweet spot render resolutions is not the limit anymore, but the optics are. So while for something like landscape VR-Videos higher resolutions would still provide a huge benefit, for porn this would be a lot lower.
