I was thinking in terms of a video clip starting with a foreground object in focus and then refocusing to make the foreground go out of focus and a background come into focus. I think layering various exposures would be difficult if there is movement in foreground or background. I believe the 50mm f/1.8 several people suggested rotates the front of the lens (and a veri ND filter) when refocusing as does the 18-200mm zoom. I don't know if you can change the ISO setting if necessary prior to the shot to get proper exposure with a large aperture and bright sunlight. Maybe I'm blowing the problem out of proportion and 99% of the time shutter speed can compensate for a large aperture.