Some people on the Internet say that the film size of traditional cameras is 35mm, which is the width of photosensitive devices, while others say it is diagonal money. Which one is correct?

35mm means that the width of the film (roll) is 35mm. Because there are perforations on both sides of the film, the photosensitive area of a single image of 35mm film is 24mm*36mm (that is, the photosensitive element area of Quan Huafu digital camera now).

Note: The picture comes from the Internet.