Try really stopping down the lens on the digicam. You may also want to focus about 1/3 from the center or so: it is a trade off between center and edge sharpness. The results will never be that great, though close up lenses can be sharp they are not flat-field - it is the nature of using closeup lenses and there isn't much you can do about it. Stopping down increases the depth of field to allow you to get more of the slide in focus.