4K resolution
As noted above, DCI 4K is 40962160, while 4K UHD is 38402160, producing a slight difference in aspect ratio rather than a significant difference in resolution. Traditional displays, such as LCD or OLED, are 3840 pixels across the screen, with each pixel being 1/3840th of the screen width. They do not overlap—if they did, they would suffer reduced detail. The diameter of each pixel is basically 1/3840th of the screen width or 1/2160th of the screen height - either gives the same size pixel. That 38402160 works out to 8.3 megapixels, the official resolution of 4K UHD (and therefore Blu-ray UHD discs).
Digital cinematography
The DCI standard for cinema usually relies on a 1.89:1 aspect ratio, thus defining the maximum container size for 4K as 4096×2160 pixels and for 2K as 2048×1080 pixels. When distributed in the form of a Digital Cinema Package (DCP), content is letterboxed or pillarboxed as appropriate to fit within one of these container formats.
In the early years of digital cinematography, 2K was the most common format for digitally acquired major motion pictures however, as new camera systems gain acceptance, 4K is becoming more prominent. The Arri Alexa captured a 2.8k image. During 2009 at least two major Hollywood films, "Knowing" and "District 9", were shot in 4K on the RED ONE camera, followed by "The Social Network" in 2010. , 4k cameras are now commonplace, with most high-end films being shot at 4k resolution.