> Riechmann has expressed that poorly in his article
Not only poorly; but extremely misleading. His article causes a lot of headaches for pre-press people and for photographers. One part that is missing is about the range of the brightnesses in the original scene for ETTR to be useful. Nothing wrong with using ETTR given one knows how far shadows can be pushed and how to use exposure compensation with ETTR. We think Zone System gives a very good guideline when it comes to compensating ETTR. Never try to get details in Zone X. The limit to the upper Zone that can be captured is posed by the number of stops shadows can be pushed to become midtones.
1. Firstly the recording of the scene with the sole purpose of protecting the brightest highlight is a departure from the original concept of ETTR, although Riechmann has expressed that poorly in his article on the subject where it was first discussed. The intention of ETTR is to shift everything to the right, even though the original article is written with the quote "...bias your exposures so that the histogram is snugged up to the right, but not to the point that the highlights are blown." The intent however comes through clearly in the rest of the article where he says for example "...when you look at the RAW file in your favourite RAW processing software... ...the image will likely appear to be too light." and "...This will accomplish a number of things. The first is that it will maximize the signal to noise ratio. The second is that it will minimize the posterization and noise that potentially occurs in the darker regions of the image." So the whole purpose is to shift things to the right, not simply to blindly shift things to the left just to protect all highlights, including specular highlights, which should rightly be blown out.
2. ETTR is not a technique to use at any ISO other than the base or lowest ISO. If you are working at higher ISO, ETTR effectively lowers the ISO anyway and the best way to improve the image would be to shift the ISO rather than use ETTR. So exposing for the subject at higher ISO should be self-evident and ETTR should be ignored. It would be ridiculous to use ETTR and high ISO. That defies common sense. That is also touched on by Reichmann with this statement "...Also be aware though that by doing this you are in fact effectively lowering the ISO used to capture the image, requiring slower shutter speeds and/or larger apertures. If you are hand-holding the camera, or shooting moving objects, the tradeoff may not be worth the reduced noise level.".
So it would be good to see the scene that was shot to come up with these figures and to analyse it, because you shouldn't be shifting the subject left with ETTR, it should be going the opposite way.
The darkest black achievable with a top-flight black and white paper, optimally processed, reflects about 1/200 as much light as the pure, paper-base white: a brightness range of 200:1. Few prints will achieve this: a more realistic brightness range is between 100:1 and 125:1. What is more, there is a difference between the total range -- the purest white and deepest black -- and the dynamic range, within which there is texture and detail. This is very unlikely to exceed 100:1, and may well be more like 64:1.
Numbers and diagrams are results of experimenting, and even contain a certain experimental error of about 0.5%..1% (see for example how 18% grey translates into L=49.5 instead of being exact L=50). The text is also based on our experience in coding raw converters. All the conclusions you can easily verify in direct experiments - a camera with a spotmeter, a grey card or a sheet of white paper that does not contain optical brightner, a source of light like a flashgun, a wide dynamic range scene like a landscape in a sunny day, and Rawnalyze software is all that is needed.
Centred exposure is an exposure that places the subject into one of three zones that result in most detailed print. In other words, it is the exposure that is centred on the proper exposure of the subject rather on the preserving extreme highlights. You may want to read the complimentary article Spot-Metering: Reading Adams in Reverse Direction for the suggestions on exposure compensation.
But sometimes, figures and samples speaks more than numbers and diagrams.
Do you have any example to show ?
And if I understand, when you speaks about centered exposure, you mean relying to in camera lightmeter without compensation ?
Thanks in advance
We've choosen another option
- public version of LibRaw is GPLed as includes GPLed code
- we'll offer personalized licenses to any developer (company or person) of specific software. Of course, personalized LibRaw will not contain GPLed Coffin's code.
> Riechmann has expressed that poorly in his article
Not only poorly; but extremely misleading. His article causes a lot of headaches for pre-press people and for photographers. One part that is missing is about the range of the brightnesses in the original scene for ETTR to be useful. Nothing wrong with using ETTR given one knows how far shadows can be pushed and how to use exposure compensation with ETTR. We think Zone System gives a very good guideline when it comes to compensating ETTR. Never try to get details in Zone X. The limit to the upper Zone that can be captured is posed by the number of stops shadows can be pushed to become midtones.
See one of the next articles, http://www.libraw.org/articles/peace-in-lights.html
Just two quick comments:
1. Firstly the recording of the scene with the sole purpose of protecting the brightest highlight is a departure from the original concept of ETTR, although Riechmann has expressed that poorly in his article on the subject where it was first discussed. The intention of ETTR is to shift everything to the right, even though the original article is written with the quote "...bias your exposures so that the histogram is snugged up to the right, but not to the point that the highlights are blown." The intent however comes through clearly in the rest of the article where he says for example "...when you look at the RAW file in your favourite RAW processing software... ...the image will likely appear to be too light." and "...This will accomplish a number of things. The first is that it will maximize the signal to noise ratio. The second is that it will minimize the posterization and noise that potentially occurs in the darker regions of the image." So the whole purpose is to shift things to the right, not simply to blindly shift things to the left just to protect all highlights, including specular highlights, which should rightly be blown out.
2. ETTR is not a technique to use at any ISO other than the base or lowest ISO. If you are working at higher ISO, ETTR effectively lowers the ISO anyway and the best way to improve the image would be to shift the ISO rather than use ETTR. So exposing for the subject at higher ISO should be self-evident and ETTR should be ignored. It would be ridiculous to use ETTR and high ISO. That defies common sense. That is also touched on by Reichmann with this statement "...Also be aware though that by doing this you are in fact effectively lowering the ISO used to capture the image, requiring slower shutter speeds and/or larger apertures. If you are hand-holding the camera, or shooting moving objects, the tradeoff may not be worth the reduced noise level.".
So it would be good to see the scene that was shot to come up with these figures and to analyse it, because you shouldn't be shifting the subject left with ETTR, it should be going the opposite way.
Roger W. Hicks writes:
The darkest black achievable with a top-flight black and white paper, optimally processed, reflects about 1/200 as much light as the pure, paper-base white: a brightness range of 200:1. Few prints will achieve this: a more realistic brightness range is between 100:1 and 125:1. What is more, there is a difference between the total range -- the purest white and deepest black -- and the dynamic range, within which there is texture and detail. This is very unlikely to exceed 100:1, and may well be more like 64:1.
(http://www.rogerandfrances.com/photoschool/ps%20subject%20brightness%20r...)
Numbers and diagrams are results of experimenting, and even contain a certain experimental error of about 0.5%..1% (see for example how 18% grey translates into L=49.5 instead of being exact L=50). The text is also based on our experience in coding raw converters. All the conclusions you can easily verify in direct experiments - a camera with a spotmeter, a grey card or a sheet of white paper that does not contain optical brightner, a source of light like a flashgun, a wide dynamic range scene like a landscape in a sunny day, and Rawnalyze software is all that is needed.
Centred exposure is an exposure that places the subject into one of three zones that result in most detailed print. In other words, it is the exposure that is centred on the proper exposure of the subject rather on the preserving extreme highlights. You may want to read the complimentary article Spot-Metering: Reading Adams in Reverse Direction for the suggestions on exposure compensation.
But sometimes, figures and samples speaks more than numbers and diagrams.
Do you have any example to show ?
And if I understand, when you speaks about centered exposure, you mean relying to in camera lightmeter without compensation ?
Thanks in advance
We've choosen another option
- public version of LibRaw is GPLed as includes GPLed code
- we'll offer personalized licenses to any developer (company or person) of specific software. Of course, personalized LibRaw will not contain GPLed Coffin's code.
It can be LGPLed with a simple caveat for the GPLed Foveon module.
By the way, I wrote about Libraw before you did it, here :
http://photofeedback.blogspot.com/2007_02_01_archive.html
Edmund
Edmund,
LibRaw uses GPLed Foveon decoder from dcraw.
-- Alex
Libraries should be LGPL licensed -
Edmund
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