Hi again!
I asked Brian Griffith, author of Raw Developer, to have a look at this topic. He answered promptly and extensively as usual. His answer is below, preceded by his authorization to me to post it here:
"No problem posting this information. The only note I'd add is that I personally have no internal knowledge of exactly what Adobe is doing inside their products or precisely how the exposure slider functions in ACR/Lightroom, but I think my description is roughly accurate based on general observations..."
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"I'm unable to see any of the images you are discussing on the forum, so I'm not 100% certain what you are going for, but hopefully this addresses your question?
Basically you can assume clipped values are either 1.0 or infinity. Most all image editors (RAW Developer, Capture One, Aperture and even Photoshop itself when not using ACR) assume the 1.0 convention when adjusting exposure. Therefore a clipped value at say -1EV exposure will have a value of 0.5 its original value. In 8bit for example a value clipped at 255 would become 255/2 or 128 rounded up as long as the color values are strictly linear (note that most RAWDev camera curves are not strictly linear and roll off the highlights... also note that many digital cameras may have some "head room" or baseline exposure adjustment and there may be valid image data above what is displayed as 255 at exposure of 0.0, also highlight recovery is implemented in many RAW editors including RAWDev which may alter the handling of clipped values).
There's no way to know whether a clipped value is just exactly clipped or may be 100x beyond the clip point or somewhere in between. Assuming clipped values are 1.0 allows you to preserve linear tonal relationships across the full color range when adjusting exposure, as the identical exposure correction is applied to every value, and this really is the technically proper exposure adjustment. The tradeoff for this is [that]clipped white values, that you may want to assume are infinitely white or at least very much brighter than their peer, become dark gray when adjusting exposure significantly down.
As you have found, Adobe's RAW software (ACR and Lightroom) handle exposure differently, more like assuming clipped values are infinite. The problem with assuming clipped values are infinite is that in order to not give an ugly appearance when adjusting exposure down, values just below the clipping point and some range below cannot have the exposure value fully applied. For example assuming 8bit values again if you had a value of 254 that was not clipped and set exposure to -1, that value would become 127 and say those very bright, but not clipped pixels were from some clouds that were right up against some sky that was clipped at 255 and was assumed to be infinite. You basically would end up with very dark clouds right up against a bright maximum white sky.
So if you assume clipped white values are infinite, in order to give a pleasing appearance you basically have to use a curve or non-linear exposure adjustment. This is essentially what Adobe does with ACR and Lightroom exposure. The exposure slider isn't really a true "exposure" adjustment equally applied to all values at least when using negative values.
In RAW Developer, to give a similar result you would need to use a manually created curve where the white point is kept at the maximum or close to it and bright highlights are rapidly rolled off (plunge downward). Over most of the image tonal range the curve would be linear say to give a -2.0EV adjustment the output values would be exactly 1/4 the input values...
Here's a visual example, not the greatest example but I had this image sitting on my desktop..., of a -2.0EV adjustment and the differences between RD and ACR visually and how a curve (very, very quickly thrown together here...) can approximate the behavior of ACR's exposure slider:"
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