Re: Media Testing for maclife.de
Re: Media Testing for maclife.de
- Subject: Re: Media Testing for maclife.de
- From: Uli Zappe <email@hidden>
- Date: Thu, 11 Sep 2008 16:30:19 +0200
Am 11.09.2008 um 15:21 schrieb Eric Chan:
The documentation available for the DNG Profile Editor says nothing
about allowed illuminant tolerances.
Well, it all depends
This is almost always true. :-)
As I'm sure you realize, there are many, many (well, infinitely
many) spectra that can map to the same correlated color temperature
value.
Yep. So it would be all the more useful if there was some
documentation about what Adobe considers usable illuminants for their
DNG Profile Editor.
I have a standard household incandescent bulb here that measures
2850 K (pretty close to illuminant A) but also a Home Depot compact
fluorescent "warm" bulb that also measures about 2850 K. They have
completely different spectra and if you visually compare a
ColorChecker (or any reasonable target) under the two lights, they
will look very, very different. So, clearly a profile built based on
one of those is going to have some difficulties with the other.
Obviously, a fluorescent bulb would be a poor choice to use with the
DNG Profile Editor.
Halogen bulbs, on the other hand, have a very smooth spectrum (the on
I used definitely had).
So would you say a studio flash and a halogen lamp, both within 100 K
deviation from 6500 K and 2850 K respectively, are suitable choices to
use for the DNG Profile Editor in a "normal" usage scenario?
There is the concept that if you shoot a target very, very
carefully, with a single light, away from all other color
influences, minimizing reflections, etc. then a profile built from
that capture will work well under all lighting. That is largely true
as long as the spectra of the test lighting is closely related to
the spectra of the training lighting (i.e., the light used to shoot
the target), and it holds true for many flavors of natural daylight,
with its fairly smoothly-varying spectrum.
This is what my measurements seem to confirm, even fluorescent light
is handled reasonably well.
If this was really true in general, however, we should expect nearly
identical behavior when analyzing the white-balanced raw data (i.e.,
in camera-native linear coordinates). That is, shoot a raw file
under one lighting condition, shoot the same target under another
lighting condition, and the white-balanced raw values should be
pretty close. Unfortunately, in general they are not.
It's clear that such a way of profiling a camera would be an
approximation only, but it seems to be one working well enough for
many usage scenarios in praxis.
Bye
Uli
________________________________________________________
Uli Zappe, Solmsstraße 5, D-65189 Wiesbaden, Germany
http://www.ritual.org
Fon: +49-700-ULIZAPPE
Fax: +49-700-ZAPPEFAX
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