sport shots and depth of feild

Hello, I have a question about shooting a softball game.
I am about 50ft away and shooting the batter and catcher. I want to blur out the fence and people behind the fence. What is the best way to do this. I am using a 70-200mm 2.8L lens. Any comments would be greatly appreciated.

The biggest question will be what focal length you can get away with. If you go to about 150mm and f/4 on the aperture, that'll give you about 5 feet of acceptable focus which should get the batter and catcher in focus but leave the fence blurred. If 150 is not long enough and you have to go to the full 200, then you'll want to drop down to about f/7 (depending on your camera this maybe 7.1 or something in that range). That should give you a comperable 5ft in you DOF.

Determine your required focal length (based on how much cropping you can get away with, which of course is dictated by the size and quality of pics you need). From there use a DOF calculator http://www.dofmaster.c​om/dofjs.html to figure out the rest.