Along with
lens flare, smart blur is one of the two most common legacies
Photoshop has inflicted upon the world of design. No longer do
Playboy's designers have to wield the airbrush; instead, they can apply
smart blur and achieve the same results in a fraction of the time. It is the modern-day equivalent of smearing vaseline on a UV filter.
How does it work? Simply put, it detects the edges in an image and applies a blur effect to everything that is not an edge. The amount of blur and the threshold whereby an edge is detected can be altered, and indeed there are options to 'invert' the effect by blurring just the edges.
It is perfect for smoothing out spotty backgrounds and averaging out skin tones, hence the reference to Playboy magazine above. The end results look airbrushed, and vaguely similar results can also be achieved by applying some of Photoshop's painterly effects. For real-world examples, Harrison Ford's mug in the poster for 'Air Force One', the covers of Pulp's 'This is Hardcore' and the UK version of Shania Twain's 'Come On Over' uses lots of smart blur, as indeed do all Playboy photographs from 1995 or so.