ABSTRACT
Create a Levels adjustment layer. Click OK without making any adjustments (we need the mask rather than the adjustment that this layer offers). From the Select menu choose ‘All’ (Ctrl+A). Choose ‘Copy Merged’ from the Edit menu (this copies everything that is visible rather than everything that is on the current layer - very useful, as there are no pixels on the adjustment layer!) Now if we select ‘Paste’ from the Edit menu the pixels are not pasted into the layer mask (even though it has been selected) they are pasted into a new layer. To paste them into the layer mask we need to hold down the Alt key and then click on the layer mask. This is the keyboard shortcut for viewing the contents of the layer mask. Only in this state will Photoshop allow us to paste the pixels from the clipboard into the layer mask.
