Unbiased noise test.

Started Feb 28, 2009 | Discussions thread
Iliah Borg Forum Pro • Posts: 25,769
Re: Unbiased noise test.

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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