From: Larry Colen
In my typical geeky fashion, I'm trying to wrap my head around all of
the ramifications of adjusting sensitivity (ISO) on my camera. Please
correct the errors in my understanding.
In the simplest form, it is a measure of how many LSBs per photon (or
tens, thousands or millions of photons).
If the base is 10 photons, per LSB at ISO 100, 1,000 photons hitting
a sensor site gives the value 100, at ISO 200, it'll give 200, and at
6,400 1,000 photons will cause the raw fill to read 6,400 for that
sensor site.
In a similar vein, if we have 14 bits of data, and at base ISO (100),
14 stops of dynamic range. That gives us a maximum data value of
16,384 which means at the above sensitivity, 163,840 photons will
cause the sensor to read maximum value, and any more than that cause
data to be clipped. If we then increase the gain by a factor of two,
then at ISO 200, we are expressing 13 stops of dynamic range in 14
bits of data, 5 photons will cause an LSB worth of change in the
data, but it will only take 8,192 photons to clip in the data path.
Since the data width is constant, every time we double the
sensitivity, we force ourselves to only use the lower, noisier, half
of the signal. The benefit of this is that we can read that narrow
band of the the sensor with more resolution so at ISO 12,800, rather
than only having 7 bits of data for the lower 7 stops of dynamic
range, we have 14 bits of data to work with. This is why using high
ISOs give us more contrast.
It seems to me that if we are shooting a low contrast situation, such
as clouds on a grey sky, or with a mediocre, low contrast lens, we
could compensate by using a higher ISO to spread the fewer stops of
dynamic range in the input out over more bits of data, at the cost of
more noise, because we're constraining ourselves to the lower,
noisier portion of the signal.
Is this basically accurate?
IMHO, you're OVER-thinking this. Just go out and look for beautiful
pictures. Take 'em.
Bracket like hell!
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