How are old movies made into blu ray?

They shoot the negatives of each frame with a high resolution digital camera

While this is kind of true, it's more wrong than right. The machine used is called a film scanner or telecine (or sometimes datacine but that term never really caught on). It does include something that's similar to a camera, in the sense that it has a sensor that detects light, but it's not a camera like the camera in your phone. It's a special-purpose scanning device that scans each frame of film, typically either at 2K or 4K resolution, and writes out a digital computer file for that frame. The result is a collection of thousands of computer files making up what's called an image sequence. This image sequence is often referred to as a "digital negative" of the film.

Ultimately, if a movie wasn't shot in high definition, you can't "upres" it to be high resolution

That's not actually right at all. Movies shot on film are much sharper than high definition. A high definition frame is 1920 pixels wide by 1080 pixels high; the lowest resolution one gets out of a film scanner, when one's in a hurry and doesn't care about quality, is 2K, which is about 2048×1556, depending on the film format being scanned. More often the frames are scanned at 4K resolution, which is twice as big a raster both horizontally and vertically. Sometimes 35mm frames are scanned at as high as 8K. There's no theoretical limit to the resolution at which you can scan a frame of film, being as film frames are analog and have no limiting resolution to them. The practical limit is that, depending on what kind of film it is, eventually you reach a point where you're just more and more finely resolving the grain.

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