[Added after the original post. Eric Chan has informed me that there are two image-processing pipelines in Lightroom: output-referred, and scene-referred. Raw files get the scene-referred pipeline. Integer TIFFs get the output-referred pipeline. Therefore, the TIFF test images are getting a different set of processing than LR applies to raw files.]
I can’t just leave you with a whole bunch of unexplained graphs, can I? In order to make sense out of all the data on color errors when Photoshop Exposure adjustment layers and Lightroom Exposure control tweaks are used to correct underexposed images, I’ve computed the CIEL*a*b* Delta E’s for each of the 121 color patches in the test image and averaged them, giving an average Delta E for each program for pushes between zero and four stops.
Here’s the graph:
I’ve plotted the results for both Process Version 2012 (the current Lightroom algorithm) and Process Version 2010 (the Lightroom 3 algorithm). There isn’t much to choose between them.
To get the curves, I corrected all the values in the sample images by dividing them by the ratio of the mean of the luminance values in the sample image and the mean of the luminance values in the test (exposed for a 0 EV push) image. That corrects for Exposure slider settings that aren’t quite right.
The Lightroom color errors aren’t huge, but they are significant. They make me think twice about blithely underexposing and pushing hard in Lightroom. The Photoshop Exposure adjustment layer seems to work virtually perfectly.
I would expect Adobe Camera Raw to perform like Lightroom.