Chuck Norcutt wrote:
> I could see that it was wrong without pushing since what should have been
> varying dark gray values were almost all zero immediately after opening the
> file in ACR. There is clearly an ACR bug and it occurs immediately on
> opening the file... no alterations required. The question is whether this is
> limited to the 5DMkII.
>
I've been reading, testing and thinking. I think I understand what's
happening. whether I can explain it clearly or not ...
First, take a look at the two histograms under the heading "Linear
Histogram", about 30% down this page.
<http://www.guillermoluijk.com/tutorial/dcraw/index_en.htm>
Fig. 5 shows the linear data from a sensor. F-stops involve
doubling/halving the amount of light for each full stop, the legend on
the bottom shows how that plays out. Let me put it in numbers, adjusted
from the 12-stop range from a 12 bit sensor to the 14 bits of the 5DII.
The top stop has a range of 8,182 values in which to store levels of
brightness and:
14 8192
13 4096
12 2048
11 1024
10 512
9 256
8 128
7 64
6 32
5 16
4 8
3 4
2 2
1 2
So - When the subject is underexposed by four stops, the number of
possible values goes from 16,384 down to 2,048. Not so bad, 8 bit is
only 256, right?
Ah, but look at the shadows. For a correctly exposed shot, there are 32
values to hold all the data for the bottom 5 stops. No matter that it
looks like more after gamma adjustment, it's still those few values
up-sampled.
In the 4 stop underexposed version, data from some of the higher stops
have to be squeezed down into those 32 values. So, potential data detail
is lost, compared to proper exposure. This is true even if the sensor
can perfectly discriminate the actual values way down at such low levels
- the data is lost to the digitization process.
To the extent that the sensor loses effective precision in the dark,
things get worse. Think of a digital meter. It's not uncommon to have
finer resolution than precision. The meter reads to 0.01, but the
accuracy is ±0.03. so a reading of 1.35. although it looks accurate on
the LCD, is really somewhere between 1.32 and 1.38. When you get down to
very low readings, the precision means values may be 100% or more off
the indicated value.
I assume the same holds true for values out of the A/D converters in
cameras.
A shot that is intentionally heavily underexposed has its true shadows
recorded in only a handful of values and its lower mid-tones compressed
into maybe 100 values. Then in the RAW converter, we try to pull those
values back up the where they would be in a proper exposure. That means
lots of big up interpolation.
To the extent that the sensor system is differentially non-linear
between channels at very low light levels, the reconstituted image will
have inaccurate color. To the extent that quantization, what might also
be called pixelization or rounding effects as analog values were
digitized down at the bottom, there will be color/luminance irregularities.
Why then, does Canon's DPP do a better job in this test than ACR? I
think the answer is simple. First, you should read on down in the page
linked above, on through the "Sensor Saturation Level" section. It may
make your head hurt, but will give an idea why the maker of a sensor
system, with full in-depth knowledge of its characteristics, can
properly fine tune conversion settings for it.
Adobe, on the other hand has to set it's default conversion behavior to
accommodate a vast array of sensor systems. Sure, they can tune settings
defaults for each camera based on some test shots, but they can't really
know what's going on down in the depths of each system.
I tried converting CH's sample color chart image in DPP and ACR. I then
tried the white balance dropper on various gray steps. In both cases,
there was a difference between the bright and darker patches. the
lighter the patch, the more accurate looking the color.
Overall, as expected from the above, the DPP conversion was better than
the ACR, and the difference became greater the darker you went. The
darker ACR patches had a distinct magenta cast, Which makes me think it
may have something to do with the effect on relative sensitivity at very
low light levels of the use of 2 green sensors for each single red and
blue sensor - or not.
My conclusions?
1. It's foolish to expect recovered shadows to be as color and luminance
accurate as if they were mid-tones. The technology just doesn't support
it. HDR image file formats use floating point decimal numbers. If future
cameras adopt a floating point RAW format, this would be different. For
now, we have to develop shooting techniques to accommodate these
limitations of our cameras to our photographic purposes.
2. There probably isn't a programming error in ACR. It just doesn't know
as much about particular sensor systems as do the manufacturers.
3. I may try using DPP for images where I care about color accuracy and
must pull shadows up a lot. On the other hand, the default color results
I get from ACR on normal 5D RAW files look more color accurate to me
than the defaults from DPP and ACR gives me more and more useful
controls. So I'll stick with ACR for most work.
Remember, most of my shooting is of natural subjects, where there is no
absolute measure of color accuracy. In many cases, the shadows in the
subject are dark enough that my own eyes are running mostly on B&W
cones, so even if I had perfect color memory, I might not know what
color those shadows 'really' were.
4. While all this is interesting to me in a theoretical way and I've
learned a lot that may help inform my image creation and processing, I
don't see how it has had a practical effect on the vast majority of the
images I've processed. Even where it may have, I'm not sure how I could
tell.
So I take a sunset shot with underexposed foreground, pull it up and get
some magenta shift. There's a lot of magenta in the shadow areas of
sunset scenes as I see them with my eyes. How can I tell the difference?
Why should I care? If it looks right to eyes that have seen many
thousands of sunsets, why worry? It it looks too magenta, I can just
pull magenta saturation down a bit. There really is no absolute in most
photographs.
Moose
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