Nils Frohberg wrote:
> On Sat, Jun 09, 2007 at 08:42:23PM -0700, Jay Drew wrote:
>
>> I read Moose's post to mean that you take a shot of an IT8 target with the
>> negative film, not that there is an IT8 target source as a negative film.
>>
>
> Missing out on the rest of the thread, I just wanted to note that this
> method is a bit flawed IMO.
Do you literally mean in your opinion, i.e. as a matter of personal
logic/speculation? Or as a result of personal experimentation? Or from
some other source to which you can refer?
> The problem is that you won't get CN's dynamic range filled up with a single
> reflective IT8 target (for flatbed scanners, e.g.).
>
Wolf Faust dies make targets specifically for use with cameras vs.
flatbeds. I don't believe they differ in dynamic range, though.
> With this method you can only measure a certain interval of your CN
> film. There are two things you can do now:
>
Here, I disagree, not as a matter of experience, or of knowledge of how
the standard is defined and works, but as a matter of logic and how I
would have dealt with the limitations of a reflective target if I were
designing an international standard.
I would start with the color space that may be reproduced by reasonably
good color paper and design a series of color patches that cover that
gamut and gray scale patches that fill the dynamic range of the paper. I
now have a reference standard of patches covering the central majority
of the larger space of all theoretically possible colors.
The process of creating a table of differences between the references
and the output of film/sensor/scanner/whatever is naturally limited to
the range of the samples. HOWEVER, it need not, and should not, assume
that the device reading it cannot exceed those limits.
Remember that this model of profiling relies on measuring differences
between devices and an abstract model of all possible colors that
exceeds the range of any known physical systems
> Either you assume your subject lies within the intensity range of
> the IT8 target. You simply cut off the blacks and whites of all your
> subsequent shots, loose that shadow/highlight detail, but gain color
> accuracy.
>
Making a highly simplified example for discussion, assume 8 bit B&W: Say
the scan of the IT8 target image on film finds the white patch at 225
(after correction with the file that comes with the target) and the
black patch at 25*. You are assuming that the profile is built to map
the white patch to 255, black to 0, etc. And that would indeed give the
result you propose.
I don't know how this stuff works down inside, but that scenario doesn't
sound consistent with the methodology of the whole international color
standards and profiling paradigm. I would expect the profiler to compare
each gray scale step to the standard value for that step ad write the
difference into the profile. So in my example, if the standard value for
white on a reflective IT8 target is 235, the profile would inform any
application using it to remap a value of 225 from the film to 235 for
display, printing, etc. If the black standard were 28, a similar mapping
of 25 to 28 would occur there. The software would have to interpolate
between values, but there are enough steps that even simple linear
interpolation should work fine.
This does leave open the treatment of values on scanned film that are
above or below the limits of the profile. I suspect that the
international standards creators have a standard for this for a couple
of reasons. First, they are smart enough to see the problem should have
a resolution. Second, they are smart enough to know that the more nooks
and crannies they can find that need definition and standards, the
longer they remain on the payroll and staying in the sort of nice
locations chosen by international standards organizations (ISOs, sound
familiar?) for their work.
I can easily imagine a few ways to do this, but have no idea what they
may have done. Nor do I know whether any or all software apps that use
icc profiles use the same method.
> Or you take various exposures of the target, with different
> shutter/aperture settings, resulting in several shots ranging over the
> entire dynamic range of your film. You can now calculate a color profile
> for the full dynamic range.
>
Something like this would indeed be necessary if your above assumption
is correct. For it to be correct does posit a level of technical
incompetence and general lack of intelligence that I find unlikely even
given the politics and general foolishness that are a part of the
establishment and staffing of any ISO.
As a practical matter as one who has regularly used icc film profiles
created in this way for some time, I haven't seen a problem. Using
profiles has made such a HUGE improvement in scanning results that I've
been more inclined to just get on with the work than pick theoretical
nits. And it just doesn't seem like I am losing any highlight or shadow
detail compared to before the use of profiles. Hmmm, I wonder if I saved
the RAW scan files from the roll that I scanned with and without
profiles....
Moose
* Assuming for simplicity that the scanner is capable of capturing the
whole range of densities on the film. This is not a practical problem
with color neg film, as the density range is considerably smaller than
for slide and conventional silver B&W film, due to the orangeish mask in
CN film. CN captures a wider range of subject brightness than reversal
film, but maps it into a smaller film density range. As an aside, this
means that the search for the scanner with the highest DMax is not
important for CN film, but hardware resolution of density differences
and a bit depth of output to contain that detail that are more important.
It also assumes that the scanner is profiled, which is at issue in other
parts of this thread, but not at issue in this theoretical example.
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