4K is not equal to 4K

What does it mean? Well, I guess anybody know what K stands for

wikipedia page for kilo (https://en.wikipedia.org/wiki/Kilo-)

Kilo is a decimal unit prefix in the metric system denoting multiplication by one thousand (103). It is used in the International System of Units, where it has the symbol k, in lower case.

(I must add also, from the same page, “1024 bytes are defined as one kibibyte (1 KiB)“)

So, what does 4K mean? It should simply mean 4000 – or four thousand. But when referring to video, it should refer to 4000 pixels of horizontal resolution – where 2K should refer to 2000 pixels, 8K to 8000 pixels and so on; the article took in account only 4K, as 2K has not the same strenght as buzzword, and still too early to broadly talk about 8K; still, they are involved, too.

OK, back on topic: 4K=4000, hence in video world 4K=4000 horizontal pixels; hence, a 4K video has an horizontal resolution of 4000 pixels, right? Wrong! (most of the times, at least…)

Let’s go back to the “old” high definition times; there were two main resolutions, 1280×720 and 1920×1080; both defined as HD, but to make a distinction between them, the latter was usually called Full High Definition, or simply FHD. The UHD, meaning Ultra High Definition, takes the High Definition to the next level; it actually refers (again) not to one single resolution, but two: 3840×2160 and 7680×4320 (four and sixteen times FHD resolution).

Now, 3840×2160 is UHDTV-1 while 7680×4320 is UHDTV-2; at the beginning, UHDTV-2 was not available, and all displays around were only UHDTV-1, so someone started to call it simply UHD – because, you know, it’s easier to spell “u-aitch-dee” instead of “u-aitch-dee-tee-vee-one” (also, is it aitch or haitch)?

But UHDTV-1 is also known as 2160p and 4K UHD; so, why don’t we get rid of the UHD entirely, and not use 4K alone? It’s just “four-kay“, even easier than “u-aitch-dee“… so, nowadays, if someone writes 4K most of the times it refers to UHD 3840×2160. But it’s wrong, at least partially.

4K should not be used alone, because it does not refer only to 4K UHD, but also to DCI 4K; the latter refers to 4096 pixels wide image – where 1K is intended as 210 and not as 103. DCI 4K full container is 4096×2160, DCI 4K scope is 4096×1716 and DCI 4K flat is 3996×2160, while DCI 2K is 2048×1080, DCI 2K scope is 2048×858 and DCI 2K flat is 1998×1080; no DCI 8K – at least so far.

You can see that, even in the worst case, as DCI 4K flat, we have 3996 pixels horizontally, or 99.9% of 4000 – and we have 4096 pixels in the other cases, or 1.024% of 4000. To me, DCI is real 4K, while 4K UHD with its “mere” 96% is “almost there 4K” – but I guess nobody will start to call it like that!

So, why should someone care about this, where 4K UHD is everywhere – UltraHD Blu-ray, web, UHD TV etc. and DCI 4K is not present in our homes? Well, to be honest, there are (very few) DCI 4K projectors available for home use – namely JVC, Sony and few others – along with very expensive monitors; but yes, they are in vast minority.

Still, I fight for the rights of the minorities; if you are like me, please start to call 4K UHD as it, and not simply 4K – because every time someone writes 4K to mean 2160p, a filmmaker out there starts to cry…

Some links:
Picking the Right Aspect Ratio – https://simpledcp.com/picking-right-aspect-ratio/
When 4K is Not Really 4K – https://tftcentral.co.uk/news/when-4k-is-not-really-4k
4K vs. UHD: What’s the Difference? – https://www.extremetech.com/extreme/174221-no-tv-makers-4k-and-uhd-are-not-the-same-thing
DCI specs – https://www.dcimovies.com/archives/spec_v1_2_No_Errata_Incorporated/DCIDigitalCinemaSystemSpecv1_2.pdf

EDIT: found this on an official SMPTE document:

“4K”: A term used to describe images of 4096×2160 pixels although sometimes applied to UHDTV1

images of 3840×2160 pixels. This term should not be used when referring to UHDTV1