PanoTools:
Re: Re: dynamic range and noise (yet again)
hugh crawford 2005-Oct-28 04:43:13
The fixed noise you could remove with dark frame subtraction
As far as I know the random noise is where there are pixels that are
brighter than they should be ( thermal noise AKA Johnson-Nyquist noise) and
thermal noise never causes pixels to be darker. What I would do is take a
group of exposures and discard all pixel values that are greater than the
median, and then average the values that are less than the median. Depending
on the noise characteristics of the sensor , the temperature , and length of
the exposure you may want to move the cutoff point for discarding values.
I have taken two exposures and combined them in photoshop using "darken" to
get whichever of the two values for any pixel is lower.
On 10/27/05, Erik Krause <#removed#> wrote:
>
> On Thursday, October 27, 2005 at 12:32, Paul D. DeRocco wrote:
>
> > > However, the technique has it's limits: Camera shake, moving objects
> > > and so on. And of course it's easier to extend dynamic range by
> > > blending multiple exposures: To gain one f-stop dynamic range you
> > > need to double the number of images.
>
> > No, you have to quadruple the number. The sum of the image data goes
> > up by a factor of four, and the sum of the uncorrelated noise goes up
> > by a factor of two, yielding a factor of two S/N improvement.
>
> Yes, of course, thanks for correction.
>
> best regards
> --
> Erik Krause
> Resources, not only for panorama creation:
> http://www.erik-krause.de/
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