How is ISO determined?

[bigblue1ca]

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[neuroanatomist]

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Or, you could click HERE for the pdf.

As for, 'how does Canon know that ISO 102400 is one stop more than ISO 51200', put in lay terms, it's simple math. The sensor detects photons of light, each pixel 'counts' those photons for the selected exposure time, storing them as electrical charge. If the ISO is set to 100 (let's assume that's the 'base' - any sensor has only one base ISO), that amount of charge is converted to a digital signal of a certain numerical value, which is the value assigned to that pixel for brightness (the position under the RGB mask determines the color that's assigned). If the ISO is higher than 100, up to the top 'native' ISO (i.e. non the expanded 'H' or 'L' settings), the sensor applies an analog gain to the signal prior to converting it to digital. Analog gain is like 'turning up the volume' on the signal electrically, and the amplifiers are precise, unlike turning the know on your stereo, it can apply a 2-fold amplification for ISO 200, a 16-fold for ISO 800, etc. If you've selected an expanded ISO like ISO 102400 on the 1D X, that is a digital gain applied after the analog signal has been converted to digital. So for the ISO 102400 case, the camera is applying the maximum analog gain (to get to ISO 51200), then it just takes the numerical value assigned and doubles it to reach ISO 102400.