Originally Posted by dyna mo

iso doesn't effect dof.

i was going to recommend going to a 35mm lens, on that t3i, that's = ~50mm, so you will get a deeper dof at the same aperture.

but there are many variables, how far you are from the scene, how far the subject is from the background, your focal length and f-stop.

so i would suggest doing a bit of the math on a dof calculator such as

http://www.dofmaster.com/dofjs.html and plugging in your specifics to see what your dof is, then make adjustments to any of the variables you can to create the perfect dof.