Before 4k TV’s came out people would use charts like this to figure out what size TV they need for their seating distance. These charts really are telling you how far back you need to be, to not see the Pixels.
Since 4k TV’s have such a large density of pixels, it is much more difficult for this issue to arise. You need to be quite close to a fairly large TV for it to be noticeably annoying.
The limit to which you can increase the angular resolution by stepping back depends on your visual acuity. At some point, your eyes are not good enough to distinguish all the details. Studies show that someone with 20/20 vision (or 6/6 in Europe) can distinguish something 1/60 of a degree apart. This means 60 pixels per degree or 32 degrees for a 1080p television. 4k UHD TVs double that to 64 degrees. Keep in mind that you can see a single pixel from further away (depending on its contrast with the rest of the picture).
For 4k, this distance is often too close for most people. This is because 1080p was designed around the field of view logic above. It is the cutting point at which both the optimal field of view and 60 pixels per degree meet. For lower resolutions, it meant sitting a bit further than preferable to not notice the pixels, 4k resolutions and higher give you a lot more freedom. Because of this, visual acuity isn’t really the best way to find the right distance anymore, and it should instead be used as a way to figure out the closest point you can sit to a TV without hitting its resolution limitation.
The chart also shows that a 4k upgrade is not worth it if you are sitting more than 6′ away and have a 50″ TV. Your eyes won’t be able to tell the difference. Ultra HD only makes sense if you want a really big screen and plan on sitting closer to it. (info by Rtings.com)
