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F. ISO, Gain and Dual ISO
F1. Understanding Gain as dB or ISO
dB gain is easy to understand. 0dB is no added gain
and is always the optimum setting for the best possible
image quality. Every time you increase the gain by 6dB
you are making the picture twice as bright, the equivalent
of opening the lens aperture by 1 stop. But adding 6dB of
gain also increases the noise in the image by a factor of 2.
6dB gain makes the picture twice as bright but also
twice as noisy. 12dB will mean a picture is 4 times
brighter than 0dB but also 4 times noisier and so on.
ISO can be a little more confusing. Every time you double
the ISO value you make the image twice as bright, the
equivalent of opening the aperture by 1 stop. But in addition,
every time you double the ISO value you also double the
amount of noise. ISO values are used to determine the
correct exposure for a camera when using an external light
meter. As different gamma curves require slightly different
exposure levels the optimum ISO (native sensitivity and
equivalent to 0dB gain) will vary depending on the gamma
curve used.
