Peter, no it is not a jump function. It is a smooth function that maps pure white to the rest of the curve below it. (If it was a step/jump function you would see horrible artifacts in your highlights and upper midtones!)
Eric, many thanks for your comment.
In the example posted above an initial RGB triplet of 236, 217, 141 is mapped to 160, 147, 107 by setting the Exposure slider to -0.75 (see screenshots with post # 44
). So the multiplier on the R channel is 160/236= 0.678
. The multiplier on the B channel is 107/141= 0.759
, which is higher. Hence, the channels are squeezed closer together in terms of R:B, and a corresponding loss of saturation is clearly visible as well. Unlike what I initially thought, this cannot be explained by the smoothened roll-off with negative Exposure.
Again, for this example all other tonal controls were zeroed in ACR, which is supposed to provide a "linear rendition". Point curve linear as well. ProPhoto RGB selected for output, which to my knowledge is what the RGB readings refer to. Further, the baseline matrix profile was used to avoid any related hue twists. Also, Rawnalyze does not seem to provide any indication for Raw clipping in this mid part of the sky.
So I’m still puzzled about this effect.