« on: July 08, 2014, 09:28:55 AM »
or is the base ISO amplification factor calculated by the camera manufacturer so that the exposure comes out right.
Yes, the amount of gain applied to the signal is targeted at a net brightness according to ISO setting, all else being equal. The smaller the sensor, the more gain.
See neuro's post here:
The answer lies in what ISO is...and isn't. Many people have a poor understanding of ISO, incorrectly assuming that a given ISO setting means a fixed amount of gain applied to the signal. ISO is a standard (that's the 'S' in ISO, ISO 12232 is the relevant standard in this case), and that standard effectively means that for a given exposure setting in terms of aperture and shutter speed, the resulting image will have a defined brightness. How does an image taken at f/2, 1/100 s, ISO 200 on a PowerShot S100 have the same brightness as an image at f/2, 1/100 s, ISO 200 on a FF sensor, even though the FF sensor is over 20 times larger? More amplification (gain) must be applied to the lower total signal from the smaller sensor. More amplification means more noise. Obviously, the same is true for m4/3 and APS-C relative to FF, to a progressively lesser degree. Likewise, a medium format sensor needs less amplification than a FF sensor to achieve the necessary brightness for a given ISO according to the standard, and therefore has less noise than FF.