I've been advised not to use a conversion lens with the Canon 100-400, but I'd still like to give it a shot. I understand my 30D won't autofocus with this combo, but I tried manual focus today, with no problem. I've also been warned about the softness. I still want to find out for myself. Does anyone know of a reputable source for rentals? Lensrentals.com won't rent the converter unless you also want to rent a lens.