"In case of a system such as a digital still camera which changes the
amplifier gain in correspondence with the ISO speed, the aforementioned
problem is serious. In such system, an appropriate exposure value changes,
and an output signal in an
image changes. As for the output signal, amplification of the signal is
controlled based on, e.g., the amplifier gain in consideration of the
amplitude of A/D conversion.

For example, when ISO speed=100, a signal obtained from a light amount of
0.1 luxsec is output as a 1× signal; when ISO speed=200, a signal obtained
from a light amount of 0.05 luxsec is output as a 2× signal; and when ISO
speed=400, a signal obtained from a light amount of 0.025 luxsec is output
as a 4× signal.

That is, since the occupation ratio of dark voltage nonuniformity in the
value obtained by an A/D converter changes upon a change in ISO speed
(amplifier gain), a serious problem is posed. Note that the signal may be
amplified not by the
amplifie gain but by output control of an A/D converter or the like.
However, in case of the output control of an A/D converter, it may become
harder to control the signal than the amplifier gain.

Even when the A/D converter has bit precision or broad dynamic range with
which the occupation ratio does not suffer, pixels originally having dark
currents like white spots (white scratches) cannot be covered by only dark
correction since they
suffer large fluctuation components called dark current shot noise."