All it is is just tweens with blurs. One tween will be blurred while one will be focused, and at the exact same time one will unblur while the 2nd will blur. Senario

Layer 1 with circel object is blurred (representing it is the closer object)
Layer 2 will be the square object unblurred (representing the focused object)
At the exact same time you make a key frame on frame 10 for each object and tween it to frame 20
Make a motion tween for each layered object from frames 10 to 20
On layer 1 with the circle object, scroll to frame 20 and make the blur effect 0
On layer 2 with the square object, scroll to frame 20 and make the blur (any blur strength will do depending on how much you want blurred.)

You now have a focus blur effect. The flash file for this short is in the description if you would like to reference it.