> But I do have to take AG to task for what I think is a totally erroneous
> statement below:
> ---------------------------------------------------------------
> The in-camera histogram is generated off of an in-camera
> JPEG and also shows only the primary (RGB) colors, not the derived
> colors (CMY). You can have the yellows clip and never know it.
> ---------------------------------------------------------------
> I think AG should certainly realize that the camera never creates
> "derived" colors. The camera and its displays and our monitors only
> have red, green and blue pixels. "derived" colors (like yellow) are
> only created in our brain. When we see yellow the only thing we are
> seeing is green and blue pixels (with maybe a hint of red mixed in).
> Pixels are not additive in brightness. It's impossible for the brain
> derived "yellow" to be blown if its component R, G and B pixels are not.
Ah, but you fail to take into account the capture side of things.
You've got several steps involved. You have the actual A-D conversion
where we are stuffing the analog light into a three-color sensor array
of individual photosites. Then this three-color infomation is
recombined to provide a RGB value for each individual pixel. The
pixels are then used for the RGB Histogram displayed on your camera.
Even a histogram that looks at the "RAW" file may still only see the
recombined value, not the photosite value.
Where things can get even more muddy is the algorithm used for
combining pixels. Is it a three-pixel combination? Four pixel
combination? Which three? Which four? Does it step every other pixel
or single pixel advance with back fill average between the two? Lots
of things going on there. Be VERY careful about assuming that it's a
four-pixel merge (two greens, one red, one blue), because it is
usually just a three pixel merge (one green, one red, one blue). Does
the method the in-camera processor uses the same as your Adobe
provided RAW converter? Which Adobe raw converter are you using? Which
CAMERA are you using? Is there two different spectral responses to the
greens? What is the spectral cutoff for each color of photosite on the
sensor?
Before you launch into a "AG doesn't know S***" response, you really
do need to look at this seriously because when you are up there near
saturation, there is a lot of ugly going on which very few people are
aware of. I mention "highlight recovery" because that is a tool which
really shows off that things aren't what you expect them to be. But
let me illustrate:
Let's take a real-life yellow, orange or red flower. Is a red flower
100% red? Rarely. To keep red from oversaturating, we have to pull the
exposure back a bit. Whenever we have a bright color which matches up
to one of the RGB primary colors, we are going to have to cut the
exposure back a ways from the peak in order to keep any form of
usability in the final file. Why? Shouldn't it be just right? No.
The red photosites are capturing more than just red. The blue
photosites are capturing more than just blue and the green photosites
are capturing more than just green. There is overlap in all three
colors. The spectral cut filters are not precise. So, when you
photograph that red flower that you would think is showing up in just
the red photosites, it's also showing up in the green and blue
photosites. If the in-camera raw converter is using an averaging
method for combining the photosites and if worse yet, it is heavily
weighting the green photosites for luminance, you are not going to get
an accurate representation of the actual clip points of the RGB
photosites in your histograms. This color clipping is common with
yellows and oranges. If you do this with any camera with two different
green sensels, you're going to end up with some very nasty artifacts
with all but one or two converters.
I've done a lot of controlled testing of this using the Kodak color
target and a number of different cameras (not just Olympus stuff) and
films. I would absolutely agree with the general sentiment that it is
far better to pull back an exposure with most Canon camera files than
to boost in post. The Olympus E-1 was a camera best served directly at
the proper exposure or with up to a stop boost in post. The E-3 will
fight you no matter what. The EM5 is a lot like the original E1 and
takes underexposure well. But all this testing did show that with the
exception of the Canon 5D, all tested cameras corrupted the colors in
the top half stop. The Nikons tested were the worse.
One way that this manifests itself is in skintones. Again, this gets
into spectral response of the photosites. It is very easy to clip
human skin even though none of the RGB histograms are clipping.
The point is that you really don't know what is going on unless you
can see the exact histogram of the photosites themselves AND have a
precise and complete knowledge of your raw converter. If you've ever
scratched your head and wondered why you bracket an exposure and end
up choosing the one to convert that has more histogram headroom, this
is likely why.
So, when I talk about "derived colors" I'm talking not just about the
visual interpretation of RGB, but of the original parsing of the real
world colors into the RGB color array sensor. I've said before and
I'll say it again, that the next frontier in sensor development with
be the RGBCMYW array where it seas red, green, blue, cyan, magenta,
yellow and may also include a luminance channel. When this happens, it
will be a true eye-opener of just how limiting the RGB filter array
really is. The RAW converter will bring the resulting file into a
standard 48-bit RGB structure, but getting there will give us a
massive increase in color gamut during capture. That is where these
stupid-high pixel densities will earn their keep.
But without a controlled test with a calibrated color target, you
really have no clue what's going on. Assuming that the RGB histogram
will always show clipped conditions is foolish and 100% wrong.
--
Ken Norton
ken@xxxxxxxxxxx
http://www.zone-10.com
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