system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do digital movies have such a hard time blending black and white, like when a person is in a dark room with a lantern? There are obvious rings as the colors fade into black.
gpt
That's called "banding", and it's due to compression. One of the steps of compression is to increase the length of sequences of the same color. This is done by combining similar colors into one. The combined results will have less granularity, and thus the distinct steps. 

Banding is a reasonable trade off to have a small file size. If videos aren't lossfully compressed, even a small video would be gigantic. With that said, the result can be improved drastically by doing frame-level adjustments on which color ranges to compress and avoiding ranges where a gradient is present. But that's operationally costly.