So there are a lot of moving parts to the discussion: sensor size, aperture, focal length, distance to subject, quality of light... but to "focus" on the OP's concern: my experience with add on diopters is that DOF is dramatically reduced compared to shooting the same setup without the diopter.
My keeper rate with the diopter has been low. I have been more successful without the diopter, working hard to get in close to the subject and cropping in if I need to. YMMV.
I was actually responding to hilljo88.
My keeper rate with the diopter has been low. I have been more successful without the diopter, working hard to get in close to the subject and cropping in if I need to. YMMV.