Is the python script NOT in newer versions of Poser?  I know P6 came with a script for calculating the DoF.  Poser 7 has one as well.  Under the Render / IO button.  Pretty sure all you have to do is select the camera you're shooting from, and then an object, and run the script.

There was a long thread on this subject some time ago.  Stewer, I believe, wrote the python script.

Best Wishes,
Rªnce