QUESTION: 4k 3840x2160 is only 2x more than 1080p, so it is really 2k?

Well first, the "4K" or "2K" refers to the horizontal resolution, not the vertical resolution. 

Here's a fun little fact: years ago, when we first started using Cineon/DPX 10 bit frame containers, a piece of real film would be scanned at 2k, which was 2048x1556. However, the actual "useable frame" of this was actually only 1828x1332. Note that this was also referred to as 2k (sometimes Cineon 2k), but in fact was slightly less than HD's 1920x1080. (Note also the 1334 was bigger, as real film was typically shot 4 perf, for a more square aspect ratio, despite that most was cropped and not used).

So max 4K (theater projection) is 4096x2160, 2k is 2048x1080 (smaller resolutions are actually used depending on the aspect ratio). Consumer / broadcast HD is close enough to 2K for practical purposes, and is 1920x1080. Plenty of feature films are actually shot at this "HD" resolution. Scaling wise, it is most easy to double or half an image in size, so when developing the higher rez consumer format, they doubled HD creating 3840x2160. Again "close enough" to 4K for practical purposes.

As a side note, see that DPX is a 10 bit format, while most HD is 8bit, particularly broadcast.

IMAGE FIDELITY

4K is really  a marketing gimmick - the truth is you get better image fidelity going from 8 bit to 10 bit and using a wider gamut, than you do going from 2k to 4K. When you go from 8 bit to 10 bit, you increase data rate 1.25 times. When you go 2k to 4K, you increase data rate 4 times. Yet people at home don't have screens large enough to even have a really noticeable difference at 4K, but they would see better image fidelity with a wider gamut. (Screens really need to be theater size to get the benefit of 4K). What I see instead is these 4K TVs add a very lame motion interpolation to bump the effective frame rate up to 60. Personally I find it nauseating, but consumers perceive it as "sharper" – and it has nothing at all to do with 4K.

 

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