Unbiased noise test.

Started Feb 28, 2009 | Discussions thread
natureview Forum Member • Posts: 62
Re: Unbiased noise test.

Thank you Iliah,

I will try it out, thanks again:)

Iliah Borg wrote:

4channnels - a command-line utility that takes a raw file and splits
it into four separate 16-bit grayscale TIFFs (1 per raw channel, R,
G1, G2, B).

Command line switches:

  • -s N selects N-th image from RAW with multiple images (not needed

for Nikon)

  • -g gamma correction (gamma 2.2) (I do not use it)

  • -A values autoscale by auto-calculated integer factor (not needed

for this experiment)

  • -B turn off black subtraction (not needed for Nikon)

  • -N no RAW curve

You will find it included as pre-compiled sample (bin folder) into
LibRAW distribution, downloadable from this page (choose your
platform):
http://www.libraw.org/download#beta

Mount your camera onto a tripod, use a lens of about 150mm focal
length, point the lens axis perpendicular to your monitor screen
filled with grey from a 2-3 feet distance, set aperture to f/8, focus
the lens to infinity, activate shutter priority, set the sensitivity
so that the shutter speed falls below 1/30, shoot, use 4channels,
open in Photoshop the G TIFF and note the resulting average raw value
in the green channel in the central portion of the frame (I use
picker in 101x101 pixels mode and Info in 16-bit mode). Next,
increase the exposure by dialing in +5 eV compensation to fully blow
out the green channel. Comparing the maximum value of green channel
you got for the centre of the frame from the second shot to the value
from the first shot you can calculate the grey value to which the
camera meter is calibrated.

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