If you want zoom is dificult to find wider than 16 or 17mm, the sigma 15-30 is out of order for people have experience with lens like that you have.Maybe one ghoice is to by one 20mm and one 15mm fisheye both must be prime and propably canon.I have sigma 15mm and is just o.k. but i think i give a tray to canon 15mm.I hear also good words about sigma 20mm 1,8,but i see hear have the tension to overemfasaise about third part lens and most take the 10d and throw in garbage with lens like 24-200, so the decision is yours .