I am a beginner into the technical side of photography and I have been having trouble figuring out which lens to buy, so I found this forum I thought there might be someone here who can help. I have a Canon 400D and a Canon EF-S 18-55mm F3.5-5.6 and I am looking for a wide-angle lens with a large aperture. I need something around 20mm, but I am getting confused about the whole EF/EF-S thing. Is 18-55mm on the EF-S the same as on a EF lens or are the numbers adjusted to the smaller sensor, so that it appear as 18-55mm would on a EF, or is it just the mount that is the only difference between Ef and EF-S? So to get a circa 20mm on the 400D, what lens should I buy, considering I need an aperture of F1.4 or F1.8? I have been thinking about the Sigma 20mm f/1.8 EX DG Aspherical RF, but that won't be a 20mm on the smaller sensor, right? Thanks in advance, I really hope someone her can help!