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?


The basic way of learning the ISO is using a calibrated light source (known spectral distribution and power), photohraph it (using RAW recording of course) and study the output (using RawDigger or alternative program) to find out the boundary EV at which one of channels ("green", typically) saturates i.e. looses any detail and becomes "1".

Or, you may learn it from independent laboratory which does those measurements. They also have an exhaustive description of ISO sensitivity.

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