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D70 Custom Curves and Noise

 
 
Xavier
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      07-23-2004
I have been reading more about the custom curves function on Nikon's
DSLRs and have this question:

Are the custom curves applied as part of the exposure process or are
they applied after exposure?

I am just trying to understand if applying customs curves in camera to
improve shadow detail will result in less shadow noise as oppose to
when the curves are applied post-capture in Nikon Capture or other
post-processing software to achieve the same effect. I understand that
applying curves are applied on a log-scale in camera and linearly in
the post-processing software. However, if the difference in applying
the curves in camera or in post-processing is merely in the scale in
which they are applied, then you won't get any noise improvement when
shadow details are lifted. But if applying the custom curves in camera
invovles changing the gain of the sensor amplifiers etc, then applying
the curves in camera will definitely result in lower noise in the
shadows. Can't figure out which is the case.

Ideas anyone?
 
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Jeremy Nixon
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      07-23-2004
Xavier <(E-Mail Removed)> wrote:

> Are the custom curves applied as part of the exposure process or are
> they applied after exposure?


The curve is used when converting the linear RAW data into a picture.

--
Jeremy | http://www.velocityreviews.com/forums/(E-Mail Removed)
 
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eawckyegcy@yahoo.com
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      07-24-2004
(E-Mail Removed) (Xavier) wrote:

> Are the custom curves applied as part of the exposure process or are
> they applied after exposure?


Everything before the ADC's are as linear as possible. Raw samples
from the sensor are linear samples. This is a good thing.

> But if applying the custom curves in camera
> invovles changing the gain of the sensor amplifiers etc, then applying
> the curves in camera will definitely result in lower noise in the
> shadows.


You can't "lower noise" in a single pixel! Equivalence,
www.google.com: "second law of thermodynamics". This is basic signal
processing. Pixel SNR is direct related to pixel value (ie, brighter
pixels have a better SNR). www.google.com: pixel noise CCD poisson.
Numerous relevant hits. Example:

http://www.fbw.hs-bremen.de/~olbers/.../doc/noise.htm

Changing the gain of the pre-ADC amplifiers -- even nonlinearly --
doesn't change the underlying SNR of a pixel. Your only option is to
draw a larger sample of photons, examples:

a) increase exposure (dynamic range problems can interfere)
b) group pixels within the image (basis of noise reduction algorithms,
high frequency rolloff can be bothersome)
c) group pixels between images (stacking, impractical for moving
scenes)
d) various combinations of the above (use (a) and (c) to work around
potential dynamic range problems of 'a', but you still are limited to
static scenes)

[Other sources of noise exist (amplifier noise, thermal electrons in
the sensor, etc), but the Poisson noise dominates.]
 
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