Re: Real-world RGB numbers for the ColorChecker SG
Re: Real-world RGB numbers for the ColorChecker SG
- Subject: Re: Real-world RGB numbers for the ColorChecker SG
- From: Robin Myers <email@hidden>
- Date: Tue, 21 Jun 2005 16:04:33 -0700
On 21 Jun, 2005, at 14:58, Roger Breton wrote:
I believe Mr. Miller is referring to RGB values for the gray patches
on
the ColorChecker SG. When used on some high-end cameras that allow for
tuning of the tonal reproduction curve (TRC), these values can be used
to calibrate the exposure conditions for fine art reproduction.
In this instance, the exact chromaticities of the RGB primaries can be
ignored because the assumption is made that the camera is adjusted to
produce equal RGB values for a neutral reference. The more important
point is the gamma value to use for the camera's RGB values. Using the
green channel RGB value only, adjusting the exposure to achieve a
particular value for the chart's white patch, then tuning the rest of
the gray patch values to achieve the reference value, a very
consistent
shooting situation is created that produces a more accurate profile
and
allows the profile to be used in subsequent similar shooting
conditions.
For more information on this procedure, check the www.betterlight.com
website in the Downloads/Macintosh area for the Repro Curve package.
The tables do not yet have values for the ColorChecker SG (they are
being edited right now), but you can get an idea of the procedure.
Robin Myers
Are there lot's of people who prefer this kind of RGb workflow for
digital
capturing the SG?
Yes. I presented this workflow recently at a Better Light user
conference and it was very well received. In fact, this workflow has
been in use by Better Light users for fine art reproduction for the
last several years with excellent results.
I guess the tools are not very widespread for doing this
in device-independent space. Which tools, Robin, are you implicitly
assuming
on the user's part in your description?
The main tool is a Better Light scanning back and the corresponding
ViewFinder(tm) software. Although, the values for the gray patches work
well for other cameras too and were used by Bruce Fraser to produce
values used in tuning raw file values as documented in his excellent
book "Real World Adobe Camera Raw".
Robin Myers
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