What's the difference between 2K and 4K?

We've all heard about different formats: 2K, Ultra HD, 4K, 1080… and they all refer to the resolution of an image. For example, HD is 1920 × 1080 pixels and Ultra HD is 3840 × 2160 pixels.

But what's this about 2K and 4K? Those are the formats approved by the DCI (Digital Cinema Initiatives), the body that sets digital cinema standards. That's why, when you shoot your short film, you'll sometimes see the camera say "4K DCI." Does that differ much from HD or Ultra HD? The answer is no—they differ very little.

Cinema and TV aren't the same shape

In cinema, the aspect ratio between width and height is different from television. In cinema we usually work around 1.85 to 1, while on TV it's 1.78 to 1 (the famous 16:9). That's why the cinema frame looks a bit more "panoramic" than the TV one.

Same height, a little more width

If you put an HD frame of 1920 × 1080 next to a 2K frame, which is 2048 × 1080, you'll see the 2K is just a touch wider. And the same happens between Ultra HD and 4K DCI: 4096 pixels wide versus 3840. The difference is small… but it's there.