[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.
I sent this to Eric for possible comment Saturday- Rex
Rex, Eric and I are in daily communication about this on LuLa. It turns out that there are two image-processing pipelines in Lightroom: output-referenced, and scene-referenced. Raw files get the scene-referenced pipeline. Integer TIFFs get the output-referenced pipeline. Therefore, all my test images were getting a different set of processing than raw files do.
I’ve done some testing with real raw files, and determined that Lightroom Exposure adjustments have mean errors of around 1 Delta-E when photographing the same test pattern off the monitor, and worst-case errors of about 4 Delta-E. However, there’s a lot of noise in the real camera testing, and the real situation is probably a lot better than that.
I’m trying to do some testing with floating-point synthetic TIFFs, which Eric says gets the scene-referenced pipeline. It’s taken me a while to learn enough about TIFF tags to have Matlab write files that Photoshop and Lightroom like, but I’m there as of this morning.
Everything looks great in Photoshop. However, when the files are brought into Lightroom, they are much more chromatic and brighter than they are in Photoshop. When exposted from LR as 16-bit integer TIFFs, they are still too bright and too chromatic. In addition, when analyzed in Matlab, the exported images have greatly distorted CIELab scatter plots, possibly because of gamut mapping, and possible because of something else.
I think I have to track down the source of the brightening.
Jim