This isn't how film speeds work.
They don't report how sensitive to light the film is.
They report the minimum level of light you need in order to achieve a certain level of
contrast when developed appropriately.
When the light level is lower, the details on the slide or negative are there, but they are submerged in muck. When the light levels get high enough, the details start to separate out and provide a usable image.
The speed rating is a measure of that point where the image comes out of the muck.
"muck" is, of course, not a technical term
