First of all, when we were still using film, we were typically using higher exposures by necessity (try to find a color ISO 3200 film, for example), while the sensor itself was much noisier by its nature. As a result, the photon noise was less important than the sensor noise.When we look back when we were still using film, the ISO showed us how sensitiv the film was, no matter how big the area of the film was; the same amount of light on a certain area gave the same exposure, and to expose a bigger film you needed more light.
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