Here is a simple way to adjust exposure in the f-stop increments that is second nature to photographers. The key to this technique is that the Screen and Multiply layer blending modes, at an opacity of 38%, produce an effect very similar to lightening or darkening by one full f-stop. (For half-stop adjustments, use 19% opacity, for one-third stop use 13%, and so on.) In this example, we’ll darken the image by one stop.
If you’re working on a flat file, duplicate the Background layer by choosing Duplicate Layer from the Layer menu. If you’re working on a layered file, create a new layer by choosing New > Layer from the Layer menu, then press Command-Option-Shift-E (Mac) or Control-Alt-Shift-E (Windows) to merge the contents of all visible layers into the new layer while leaving the underlying ones intact.