I can't figure this one out. Could someone explain it to me please? I have a D80. I love my 50mm but need something wider.I was considering the sigma 30f/1.4. Then I read this on photozone about the lens: "The AF 30mm f/1.4 EX HSM DC is one of Sigma's Digital Camera lenses designed specifically for APS-C DSLR. It has a reduced image circle so it's not compatible to 1.3x or full-frame (D)SLRs" What does that mean? Will it be even wider on my D80 than for example 30mm focal length from the 18-135 or just the opposite?