Subject: Re: Exposure to the right and tone placement
This thread has become overheated and I would like to return to the points I made in my original post and invite your analysis so that we can all learn. As I stated, I can't follow all of Rags' arguments and will let him explain them himself, but I thought he made some good points that were worthy of discussion.
Yes, I know what Jeff was referring to the clipping display in ACR or Levels, but it now has nothing to do with the LUT and he replies with a non sequitur rather than retracting his irrelevant statement--no one stated that ACR did not show clipping levels. For some reason Jeff just gets under my skin. His attitude is condescending and cynical.
I certainly appreciate your exposition on photometry and human perception, but I presume it was prompted more by technical misuse of some of the terms by non-experts than anything actually germane to the substance of the discussion :). By now we all should know that a digital sensor is a photon counter and is linear--that point does not to be re-iterated. Of course, Jeff will reply that I may know that, but do not understand its significance. I would rather drop this point.
That said, here is what I strive for with exposure and I would appreciate your critique. For a subject that contains zone IX values, I like the histogram in ACR to be as far to the right as possible without clipping, realizing that the camera displays may not be accurate and taking this into account. I agree with Norman Koren that one can leave a little headroom on the right, since the highlights contain an abundance of levels. However, as you point out,the effect on shadow detail and noise may be an overriding consideration. If the dynamic range of the sensor is exceeded, one must sacrifice either shadows or highlights or use HDR if possible.
I did not mean to say that underexposure facilitated recovery or preservation of shadow tones. Since there are more tones below Zone V than above it, loss of those higher tones by overexposure and clipping would be more significant than loss of equal number of the more numerous shadow tones from underexposure.
When the image is rendered from scene to print or screen tonal values, the highlights and shadows must be rolled off to fit the dynamic range of the medium while maintaining adequate contrast in the mid tones, and you lose tones in these areas anyway. I do not think it is necessary to exceed the limitations imposed human vision as dictated by the Weber-Fechner law in the final image--in other words, we do not need 2048 discrete tones in Zone IX but rather 70 or so. According to Norman Koren, fewer levels are needed in the shadow zones, due to visual interference-- mostly flare light-- from the light areas.
In your future books I hope you will cover scene and output spaces and scene to output rendering in more detail and possibly some material on tone perception.
With a low key subject with no higher tone values, I would not expect the histogram extend all the way to the right, but would want it to be representative of my visualization of the picture, making sure that the shadows are well exposed.
In summary, I would regard the exposure as proper when I obtain the intended tonal values without needing to use the exposure control of ACR. I certainly do not like it when I need to use positive exposure compensation. As an aside, Nikon digital cameras in high contrast situations often underexpose, supposedly to avoid blowing the highlights--this can be quite annoying. I wish you would talk to them. :)
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