Fake Depth of Field in Photoshop

(click on image to take a close look)

The obvious method: combine a blurry and sharp version of the same image using a layer mask. In most cases this is good enough, but look how the blur behaves through the transition.

(click on image to take a close look)

Blur through a selected alpha channel instead and the blur radius actually grows in proportion with the brightness of the alpha channel. Nice.

This 'ramping up the parameters' behavior is true in other Photoshop filters, as well.

UPDATE

Actually, Photoshop seems to be inconsistent on this issue.

Some filters, like Ocean Ripple, do not ramp up, while others, like Ripple, do ramp up their strength based on alpha.

What a mess.