You say iso has nothing to do with exposure. Ok. Set your iso to 100, go outside on a sunny day, and take a picture. Now go inside, turn off lights and close the drapes. and adjust your shutter speed and aperature to anything you like, and take a picture. (No cheating with a flash or tripod)t Is the exposure the same? No? Now adjust iso, and suddenly you can get a picture with the same exposure as the first one. Just because digital iso doesn't do it the same way as it did with film, doesn't mean it suddenly somehow doesn't affect exposure anymore. It still does the same thing. 200 iso film is twice as sensitive to light as 100iso film. A digital camera sensor set to iso 200 iso is twice as sensitive to light as a sensor set to 100 iso. A jet airliner accelerates by burning gas fuel resulting in expanding gasses pressing directly on the front of the combustion chamber, an electric car accelerates using electricity and magnetism. In both cases, it's still acceleration and accomplishes the same thing, just in different ways.