Its to do with circles of confusion, light bounces off on object and when those beams cross before they hit the sensor this causes a circle of confusion and the object to be out of focus more light coming in causes more so does a bigger distance from the focus point and magnification causes these circles to be bigger.
For a good explanation on DOF I would recomend watching this episode of meet the GIMP at the end he discusses DOF so if you dont use GIMP just fast forward to the end
Meet the GIMP » Blog Archive » Episode 130: Getting the Bugs out
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You cant fool all of the people all of the time, some of the time all of the people will some of time but not all of the time as some of the time all of the people will some of the time but all of the people will not all of the time
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