Sandman
I.e. if you open a photo in Photoshop and click the Levels button in the palette, which is the easiest way to apply a levels adjustment, it's fully reversible. Only if you select Image ->Adjustments ->Levels do you get a levels adjustments that isn't reversible.

nospam
the easiest way to do levels is pick levels in the adjustments menu which is not reversible.

Sandman
How is that the easiest way when the levels button is right there on screen?

nospam
that's new and not always visible and merely a shortcut for a new layer.

Savageduck
Yup that creates a specific Adjustment layer.

nospam
the menu choice for levels has been there forever. command/control-l, for levels.

Savageduck
Using the menu or shortcut, does not open a separate, or duplicate layer, it opens the specific adjustment dialog window, and applies the parameters to whatever layer is active.