I installed the CHDK firmware on my camera. One of the options it has is to show the "true" ISO that any given displayed ISO represents (eg. display ISO 80 is really ISO 45). This got me wondering: how can I verify this? And can I check to see if the other displayed values are real, or marketing numbers?
Measuring the shutter speed is easy: take a picture of an object moving at a known rate (say, a colored disk attached to a 60Hz synchronous motor) and see how much it blurs. But how can I measure aperture and ISO?