Moose writes:
<<Another, broader way to look at it is that forcing anything, from material to
system to person, beyond it's limits will result is failure of <<some sort. By
underexposing, then trying to raise shadows well above any actually useful
level, you forced this particular system <<beyond its limits into failure. It
happens to be banding.
Yes, Moose of course is correct that any sensor pushed hard enough for
underexposed shadows will produce some patterned noise.
(Ferengi Rule of Acquisition and Exposure #117---"You can't free a fish from
water.")
However, note Sony cams (low "N2" noise sensors) with the latest sensors seem
to become almost ISOless a few stops above base ISO:
http://www.dpreview.com/reviews/sony-alpha-7-ii/11
As MG wrote on 1/27/14:
You can simplify the noise additions to the signal in the camera into
two stages - one is close to the pixel, one is close to the A/D
conversion. The "real" analog ISO amplifier is closer to the pixel.
I'll call them "n1" and "n2" here. NB - this is not a real convention.
In low light, you need to amplify the signal to make it brighter - the
question is just where you do it.
If you do it in post, you're basically applying a multiplication to the
SUM of n1 and n2.
If you do it in the analog ISO amplifier close to the pixel, you're
just multiplying n1 - the late stage noise n2 is the same as before,
and is added as normal without multiplication!
Say we need a 10x brightening (go to ISO1000, if the camera's base ISO
is 100). To throw in some arbitrary numbers of n1=5 and n2=20 just to
see what happens.
Brighten in post:
Total noise = (brightening factor) * (sqrt(n1^2+n2^2))
>>>> 10 * sqrt( 5^2 + 20^2 ) = 206.1
Brighten in analog, with ISO amplifier:
Total noise = sqrt ( (brightening factor * n1)^2 + n2^2)
>>>> sqrt( (10*5)^2 + 20^2) = 53.9
-the noise is much lower when you use the on-sensor ISO amplifiers,
because of the fact that you don't have to multiply the late stage
noise too...
In cameras with very low n2 noise (late stage noise) you don't lose as
much by amplifying in post. Say that we lower n2 to "10" instead -
that's like getting quieter A/D converters and a cleaner signal path.
Post : 10 * sqrt( 5^2 + 10^2 ) = 111.8
ISO : sqrt( (10*5)^2 + 10^2) = 50.9
Note that the "push in post" noise got a LOT lower - but the "amplify
with analog ISO" didn't get much better at all. That's because of the
n1 having to be amplified no matter what, there's no getting around
that if the image is to get brighter.
That is (fairly simplified) why you can push a Sony sensor quite high
from base ISO without the image falling apart, but you can't do it with
a Canon sensor. The Canon sensor has good high ISO in camera, since it
has excellent n1 noise. But it's limited at low ISOs by the rather
strong n2 noise, and that's amplified by pushing in post.
This is also why Canon sensors have lower base ISO DR's, since DR is
dependent on both n1 and n2. Higher ISOs are almost purely dependent on
n1."
Solution is as per Moose and expose more and sacrifice some highlights,
exposure bracket if subject allows, or even consider
using a Sony cam with the Kolari Mod so Leica WA's work well, (gasp). Probably
wold wait to see what they do to the filter stack on the soon to be announced
A7rII or whatever they call it, actually.
No bands, Mike
--
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