I've been wondering about the possibility of shooting ultra-wide macro pictures by using extension tubes on wide angle lenses, especially on a 15mm fisheye. I haven't been able to find much relevant information about such combinations. All I've found is that a 12mm extension on an EF 15mm fish-eye was going to give me 2 to 5mm of working distance, with magnifications around 0.9x, and similar information for other lenses like my 24/2.8. Does anyone have experience using extension tubes on a 15mm fish-eye? Any comments about image quality (I'd be willing to stop down by 3 to 4 stops)? How about getting light on the subject? Does anyone have sample pictures? Thanks!