ISO vs ASA vs DIN: what's the difference?

These are the 3 most common film speed formats that you would most likely run into (mainly when picking film stocks). What are the differences? Why do they all exist? And how do we convert between them?

📲 Links:
ISO document: https://www.iso.org/standard/3580.html
📍 Timestamps:
0:00 Intro
0:20 The first standard: Leon Warnerke
1:27 DIN
2:57 ASA
3:19 ISO
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Видео ISO vs ASA vs DIN: what's the difference? канала Cathy and Leon