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Author Topic: Expose to right, it is as simple as  (Read 35152 times)
PierreVandevenne
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« Reply #220 on: September 08, 2011, 05:06:00 PM »
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That particular answer, which I also just read, strikes me as totally confusing and utterly uninformed. In fact, this is a ridiculous/hilarious statement.

The data is there: that is the data they generate the jpeg from, and then subsequently generate the histogram of the jpeg from.

So you have

-> data (let's be generous and say 16-bit values for each x y point in the array)
-> relatively complex transformation of the above data in a x-y array of 8 bit values
-> generation of an histogram of those 8 bit values

and the claim is

-> same data
-> generation of an histogram of those 16 bit values

is impossible.

Jesus! How can one be so dense (sorry)! If they want, they can generate an histogram of how many pictures users take every hour or anything else they measure.

Now, of course, if a Leica executive said it, you can be sure it will become gospel for thousands of photographers. But that is utterly absurd: gazillions of histograms are generated on a continous base by zillions of sensor users who just want ot generate histograms.

Even if you don't care a bit about the technicalities, one has got to admire the logic of the process.

"we don't generate an histogram of the raw data because the picture doesn't exist, yet susprisingly the picture exists enough to derive a jpeg from that very same raw data and then an histgram of that jpeg...."

Priceless.


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Guillermo Luijk
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« Reply #221 on: September 08, 2011, 07:20:22 PM »
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Nick Rains surely heard of histograms for the first time applied to digital cameras, and he probably isn't aware that those population pyramids he studied at school were histograms as well:




A genuine Canon 5D RAW histogram, showing non-zero black point and non max saturation point (3692). If no gamma is applied, the linear histogram is of little use to the photographer since it is concentrated on the left side due to sensor linearity:




A simple gamma curve, or even better a log representation in the X-axis, make the histogram very meaningful to the photographer.
Here RAW histograms in EV divisions for different exposures 1 stop apart (note the histogram doesn't change in shape thanks to the log scale):



A quick glance provides information about any RAW clipping, highlight exposure headroom in EV (perfect for ETTR!) and scene's DR.


Regards
« Last Edit: September 08, 2011, 07:43:49 PM by Guillermo Luijk » Logged

BartvanderWolf
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« Reply #222 on: September 09, 2011, 02:38:26 AM »
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A quick glance provides information about any RAW clipping, highlight exposure headroom in EV (perfect for ETTR!) and scene's DR.

Hi Guillermo,

I agree with an EV based representation. In addition, IMHO, a histogram for the purpose of detecting clipping issues would become even more useful if the bin counts (vertical axis) are also represented on a logarithmic scale. For determining optimal exposure I'm less interested in the bins with a high count, and more interested in the ones with low counts, e.g. for detecting the onset of clipping by specular highlights.

Cheers,
Bart
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ejmartin
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« Reply #223 on: September 09, 2011, 07:06:52 AM »
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Hi Guillermo,

I agree with an EV based representation. In addition, IMHO, a histogram for the purpose of detecting clipping issues would become even more useful if the bin counts (vertical axis) are also represented on a logarithmic scale. For determining optimal exposure I'm less interested in the bins with a high count, and more interested in the ones with low counts, e.g. for detecting the onset of clipping by specular highlights.


Agreed; it would also be trivial to give some indicators, say 0.001 fraction clipped, 0.01 fraction clipped, etc.
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emil
Peter_DL
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« Reply #224 on: September 09, 2011, 10:45:25 AM »
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But if you're using ACR or Lightroom, my personal (not official) recommendation is simply to use Exposure to compensate for whatever additional in-camera exposure you may have used.  For example, if you did ETTR by adding +1 stop of exposure at time of capture, try setting Exposure -1 in ACR/LR.  This will not result in hue shifts compared to if you hadn't used ETTR.

Wouldn't this require a profile with lightness-dependent Hue/Sat.-corrections ?

I'm referring to the first table for accuracy, such as in particular from the Chart Wizard,
not any 3D Look table which may come later on, after the Exposure slider in the processing pipe.

Inconvenient question,
or just irrelevant, or just no time for being around  Huh

Anyway. Cheers!
& Best regards, Peter

--
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NikoJorj
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« Reply #225 on: September 09, 2011, 12:06:00 PM »
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"we don't generate an histogram of the raw data because the picture doesn't exist, yet susprisingly the picture exists enough to derive a jpeg from that very same raw data and then an histogram of that jpeg...."
Grin Cheesy Grin
Yes, that's quite puzzling indeed. Why?
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Nicolas from Grenoble
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bjanes
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« Reply #226 on: September 11, 2011, 08:51:57 AM »
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I don't agree here. There will be a day when a user will say 'STOP'. You don't bracket a LDR scene, do you? even if you can afford to. When a time comes in which HDR scenes can be captured in a single shot, users won't bother to bracket (bye bye to misalignment issues or ghosting artifacts, less resources needed, handheld HDR).

The same applies to ETTR (for the same reason: sensor noise lowering). If you can afford to underexpose and still have a high quality image, you won't bother to ETTR. In fact you'll probably try not to ETTR in order to make sure highlights are preserved without caring too much about exposure. This will allow to shoot faster and concentrate on other aspects like composition.

This image was underexposed by 6 stops on a Pentax K5 (6 upper stops in the histogram with no information):

It's a extreme non-ETTR, but still the resulting image was quite OK. If underexposure had been just by 3 stops (still a huge gap from perfect ETTR), the IQ would have been the same as with ETTR in practice.


The quality of the 6 stops underexposed image with the K5 is astounding, but I don't think that is a good excuse to become sloppy in exposure. The image under discussion was relatively low dynamic range and viewed at low magnification. One must remember that for each halving of exposure, one loses 1 stop of DR and the SNR increases by a factor of 1.4. These facts are shown in measurements by DXO for the Nikon D7000, which uses the same sensor. The results are for ISO and not exposure, but these new cameras are effectively ISO-less and the results of increasing the ISO are similar to those obtained by underexposure.

For optimum results with a high dynamic range image scene, one should fully utilize ETTR. For less demanding shots, one can leave a bit of highlight headroom for safety. IMHO, these cameras with better sensor performance do not negate the benefits of ETTR, but change our approach to utilizing it.

Regards,

Bill
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Guillermo Luijk
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« Reply #227 on: September 11, 2011, 09:06:40 AM »
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For less demanding shots, one can leave a bit of highlight headroom for safety. IMHO, these cameras with better sensor performance do not negate the benefits of ETTR, but change our approach to utilizing it.

We are saying the same thing Bill, the better sensors become, the less need for ETTR. Or what is the same, the less need to bother about achieving perfect ETTR.

If my camera has an effective DR of 11 stops, and the scene's DR is just 7 stops, I could be even more interested in doing ETTR-2EV than perfect ETTR. This will give me two extra stops of shutter speed (always a good idea for sharpness), and extra safety headroom for highlight clipping at no practical IQ cost in the final image:

Camera's DR: ***********
ETTR:        ----OOOOOOO
ETTR-2:      --OOOOOOO--

« Last Edit: September 11, 2011, 09:53:40 AM by Guillermo Luijk » Logged

hjulenissen
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« Reply #228 on: September 11, 2011, 12:05:32 PM »
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If my camera has an effective DR of 11 stops, and the scene's DR is just 7 stops...
Most people, even most photographers might not care less what Dynamic Range is.

For those that do care, I suspect that they would rather have 6 stops than 4 stops of SNR in the darkest tones, if cost only a slight inconvenience.

I tend to see the whole "ETTR" thing to be a discussion on the "right exposure", not "exposing as much as possible to the right". Right now, photographers have poor in-camera feedback to tell them exactly how an image was/will be exposed, and therefore, they will tend to introduce some margins that may or may not be needed or wanted in a particular image. If and when those feedback mechanisms are improved, the photographer will have more control over the exposure. That will result in a better ability to utilize each sensor generation closer to its fullest, no matter how much progress is made in the core sensor abilities.

-h
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