Terms like "2K" and "4K" don’t refer to specific resolutions. They are resolution categories. They are used to classify resolutions based on horizontal pixel count. "2K" refers to resolutions that have around 2,000 (2K) pixels horizontally. Examples include:
1920 × 1080 (16:9)
1920 × 1200 (16:10)
2048 × 1080 (≈19:10)
2048 × 1152 (16:9)
2048 × 1536 (4:3)
All of these are examples of 2K resolutions. 1920×1080 is a 2K resolution. 2048×1080 is another 2K resolution. 2560×1440 is not a 2K resolution, it is a 2.5K resolution.
"2.5K" refers to resolutions around 2,500 (2.5K) pixels horizontally. For example:
2304 × 1440 (16:10)
2400 × 1350 (16:9)
2560 × 1080 (64:27 / ≈21:9)
2560 × 1440 (16:9)
2560 × 1600 (16:10)
All of these are examples of 2.5K resolutions.
So why do people call 2560×1440 "2K"?
Because when "4K" was new to the consumer market, people would ask: "What's 4K?", and usually the response was "it’s four times as many pixels as 1080p". Unfortunately most people misinterpreted this and assumed that the "4" in "4K" actually stood for "how many times 1080p" the resolution was, and since 2560×1440 is popularly known as being "twice as many pixels as 1080p" (it's 1.77 times, but close enough), some people decided to start calling it "2K", and other people heard that and repeated it.
While it’s true that 4K UHD (3840×2160) is four times as many pixels as 1920×1080, that isn’t why it’s called "4K". It’s called 4K because it's approximately 4,000 pixels horizontally. The fact that it’s also 4 × 1080p is just a coincidence, and that pattern doesn’t continue with other resolutions.
For example, the 5K resolution featured in the Retina 5K iMac, 5120×2880, is equivalent to four 2560×1440 screens. If 1440p is "2K" because it’s twice as many pixels as 1080p, then wouldn’t four of them together be called "8K"? (Well, technically 7K since like I said 1440p is 1.77 times not 2 times 1080p, but that’s beside the point). We don’t call it 7K or 8K. We call it 5K, because it's around 5,000 pixels horizontally. It has nothing to do with "how many times 1080p" the resolution is.
In addition, an actual 8K resolution such as 8K UHD (7680×4320) is equivalent to four 4K UHD screens. A single 4K UHD screen is four times as many pixels as 1080p, so four of those together is sixteen times as many pixels as 1080p. But 7680×4320 isn't called "16K", it’s called "8K", because it’s approximately 8,000 pixels horizontally. Again it doesn't have anything to do with "how many times 1080p" the resolution is.
So although 2560×1440 is around twice as many pixels as 1080p, it is not called "2K", because that isn’t where these names come from. Since 2560×1440 is approximately 2,500 pixels horizontally, it falls into the 2.5K classification.
Examples of How the Cinematography Industry Uses These Terms
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