MadManChan2000 wrote:
Yes, digital Exposure (in the camera) generally involves doubling the values -- until you clip. Then you have a sharp cutoff, which is very non-linear. Current sensors work that way, because they have to (not because it makes images look good). When we implemented our Exposure control in PV 2012 in software, we saw no reason to mimic that undesirable characteristic. Instead, we took our inspiration from the much gentler highlight rolloff (also non-linear) that film provides (the term "exposure" was used for many years in that context too).
Thus, both analog and digital exposure have non-linear characteristics in the highlights: we opted for the route that we feel leads to better-looking output.
This makes sense for most situations, but it can complicate ETTR rendering. If one is dealing with a relatively low contrast scene and increases the exposure by 2/3 stop to get a better SNR without encountering any clipping, a linear exposure correction of - 2/3 EV would normalize the image.
The use of the exposure and brightness controls are discussed by Thomas Knoll in the LuLa ACR turorial (CR09, 5:27). He first adjusts the white point with the exposure slider, and if this affects the midtones, he uses the brightness to correct them. He mentioned that with Lightroom (? LR3 at that time), some users set the midtones with the exposure control and then set the highlights with recovery. Setting the midtones with exposure to place them on the linear portion of the characteristic curve is what is usually done with B&W negative film. One then hopes the highlights and shadows are compressed by the toe and shoulder of the characteristic curve, perhaps altering contrast by varying the development. With ACR7, the exposure control can be used to set the midtones and the white and highlight controls can be used to set the white point and control the highlights, similar to the process with film as you suggest.
However, with ETTR, one places the highlight just short of clipping by adjusting exposure, which is a linear function. A linear exposure control would be useful to linearly decrease the image luminance values. I presume one can always fall back to PV2010, but there is no guarantee that it will be included with future versions.
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