Re: Outdoor/daylight camera profile
Re: Outdoor/daylight camera profile
- Subject: Re: Outdoor/daylight camera profile
- From: Ray Maxwell <email@hidden>
- Date: Mon, 22 May 2006 13:32:39 -0700
Michael Fox Photography News Account wrote:
I’m interested in making
a camera profile -- in
particular, a daylight profile for a digital back -- and I am looking
for
pointers.
The existing Outdoor
Daylight profile from Phase One lacks
highlight definition, saturation in light colors and have a hue shift
in the
blues toward cyan. I made a “quickie” profile which did a
much better job. By “quickie” I mean I stuck the target in
the sunlight and took a shot without worrying about such things as
color
reflection from the tan table it was sitting on, specular highlights,
etc. Even so, when comparing the colorchecker numbers in the image
using
my quickie profile vs. the theoretical/average numbers provided by
Bruce
Lindbloom and others, I got a much better match than with the supplied
profile. When applying to various images, the result was also better,
with much better highlight detail, better saturation of light colors
(like
rainbows) and no hue shift in blue values.
A note of caution...Michael you are speaking as if the camera profile
has total control of the color you perceive. Let's work through this
assumption.
The camera outputs a RAW gamma 1.0 mosaic file. Software (in camera or
external) then processes this data and outputs an RGB file in some
color space (Adobe RGB98, ProPhoto RGB). If you open it in Photoshop
the file is now converted using your monitor profile. This is your
first opportunity to evaluate the color. The data has been converted
using three profiles. It has traveled through the camera profile, the
working space profile, and the monitor profile. If you print a "proof"
the image will have to be converted again by the profile for your
specific printer, ink, paper profile.
Camera profiling is art as well as science. The first question you
have to ask yourself is, "Do I want "accurate" color or "pleasing"
color. If you have ever done careful measurements on almost any kind
of transparency film, you know that it produces pleasing color and not
accurate color.
Now let's assume that you want to do art reproductions and that you
want accurate color. Some people would start with a standard color
target such as the GretagMacbeth ColorChecker Chart. This will get you
into the ballpark. However, if the spectral characteristics of the
pigments used in the ColorChecker are not the same as the pigments used
in the artwork, you will not get accurate color since your camera is a
three-channel wide band colorimeter. It is not a 32-channel
spectrophotometer. If you want to make very accurate reproductions of
artwork you will need to make your own test chart using the pigments
that the artist used in the painting. You would then read the Lab
values (using a 32 channel spectrophotometer) of each patch and then
use this data to make a profile. Your test chart now has the same
spectral characteristics as your artwork. Now if you change artwork
from oils to water color, guess what you have to do.
What if we now go out and point our camera at the real world. We are
now pointing at reflective objects that have all different reflective
spectral characteristics. Given this, it is a matter of taste as how
we decide to render these inputs. Keep in mind that you will go
through at least three profiles before you get to see the final result.
I wish color science was simple. However, it is not. We have
automated many of the steps in making consistent color, however, in the
final case it is always a matter of taste in the mind of the viewer. I
have not even touched on the fact of viewer metamerism. This means
that you can measure as much as delta E 5 when asking two different
(non-colorblind) persons to evaluate a side by side match of color
patches made with different pigments.
Just remember you are adjusting an end-to-end system when you adjust to
suit your tastes.
I have been studing color science for more than 15 years. I had no
idea how deep a subject this was when I started.
"Beware...there be mosters here."
Ray
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