Narrow DOF

Just to clarify... To get the subject in focus and the wall/background few (1-3) feet OOF, should I user my 17-55 or 70-200 on my 40D?

Thanks for making this clear for me....

It's going to depend on if you're still shooting from the same distance away from the subject with each lens. The longer lens should give you a narrower DOF, unless you have to back up in order to use it.

Here, just plug in your settings into this DOF calculator and it'll tell you the results.

http://www.dofmaster.c​om/dofjs.html

Assuming that you want the subject to be the same size, i.e. that you will stand proportionately farther away with the longer lens, DOF will remain essentially the same. Try it at Dofmaster: Put in 50 mm. focal length and 10 feet distance. At f5.6 the DOF is 2.62. Now enter 200 mm. and 40 feet. The DOF is 2.58. The difference, however, will be that with the shorter lens more of the background will be in the picture.