What you are looking for is called Depth of Field, this is done by changing the correct attribute.
Here is one way:
And I think if you change the focal lenth of your camera then it gies the right effect. This is an extract for CGTalk about this subject:
"... create a distance mesaure tool. parent one locator to the camera and one to an object which lies in the center of your focus.
then via the connection editor, connect the distance from the distanceShape to the focus distance of your cam... "
Hope it helps.
Chris (formerly R@nSiD)
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Winner SM VFX Challenge 1
3rd Place SM SteamPunk Challenge (May 2007)