What's the best lens to shoot food with my 40D, assuming 1) I'll have to deal with low light, flashless, close-quarters restaurant situations. 2) I can't afford the new $1,700 24/1.4! I have the 17-55/2.8, which is a fine lens, but I'm yearning for something primey. Does the 28/1.8, for example, offer better IQ than the 17-55? Thanks for any suggestions you can offer.