Monkeycloud - any image adjustment in Photoshop can be applied to a selection, it's not limited to filters.

So for Paul's needs here I'd paste the zdepth mask into a new Channel, then load selection, refine mask if needed, and work on a copy of the layer (toggle visibility on & off to see if the chages are working). Then just a case of experimenting with levels, curves, brightness etc.

Also easy to add painted elements like smoke etc this way too.

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