P.S. A *very* easy way to do this is with 35mm... using a longish fast AF lens or just a manual lens with focus confirmation. E.g. the nikon 105/1.8 gives extremely shallow DOF. Let's see, I think that's what I did for this (on the left) quick handheld shot. Getting the focus right is very easy, and if you screw up one frame, hey, it's 35mm roll film, who cares. You can 'bracket' the aperture if you wish. If you do your shooting to slide (e.g. astia or scala or some other dr5 processed b&w) then you can make a 4x5 or larger dupe neg on b&w and off you go.
Beware that super-isolated DOF has become quite the fad lately, but... I'll admit it is fun to play with.
P.S. For ultra shallow DOF with no focus difficulties, I also use the Nikon 50/1.2, it's great fun.