In 2005, Digital Cinema Initiatives (DCI), a prominent standards organization in the cinema industry, published the Digital Cinema System Specification. This specification establishes standardized 2K and 4K container formats for digital cinema production, with resolutions of 2048 × 1080 and 4096 × 2160 respectively. The resolution of the video content inside follows the SMPTE 428-1 standard, which establishes the following resolutions for a 4K distribution:
- 4096 × 2160 (full frame, 256∶135 or ≈1.90∶1 aspect ratio)
- 3996 × 2160 (flat crop, 1.85∶1 aspect ratio)
- 4096 × 1716 (CinemaScope crop, ≈2.35∶1 aspect ratio)
2K distributions can have a frame rate of either 24 or 48 FPS, while 4K distributions must have a frame rate of 24 FPS. Some articles claim that the terms “2K” and “4K” were coined by DCI and refer exclusively to the 2K and 4K formats defined in the DCI standard. However, usage of these terms in the cinema industry predates the publication of the DCI standard and they are generally understood to be casual terms referring to any resolution approximately 2000 or 4000 pixels in width rather than names for specifically defined resolutions.