Thanks, folks. I think it's going to only be very approximate to attempt to show the limits of the depth of field, because it depends on final image size. Possibly the only way for computer art would be to base it on how much an out-of-focus point would bleed into an adjacent pixel. Phtotgraphy textbooks do give tables of depth-of-field for various combinations of focal length and aperture. Maybe we're looking at the problem from the wrong direction? Move the indicator plane to where you want the limits to be, note the figures, and set aperture and focal distance to suit that. This is, pretty well, what I'd do with a camera.