Limitations of Adjustment Layers
One limitation of adjustment layers prevents certain adjustments from being available as an adjustment layer: An adjustment layer must be able to be fed a single pixel and figure out how it should be modified, without having to rely on the information contained in the surrounding image. The Match Color, Replace Color, Shadows/Highlights, Exposure, and Equalize adjustments must be able to compare the area being adjusted to the surrounding image (or a second image) to determine how to adjust the image, which prevents them from being used as adjustment layers.
Five adjustments are not found in the Adjustment Layer pop-up menu at the bottom of the Layers panel: Auto Levels, Auto Contrast, Auto Color, Desaturate, ...
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