I was idly looking at the details of the Sigma 10-20 on Amazon when I saw something that didn't make a lot of sense. A couple of people had written reviews in which they mentioned that the 10mm setting was 'equivalent to 15mm' on 35mm. Firstly, I'd always assumed that when a lens for a Canon or Nikon DSLR was sold the focal length specification was given for 24x36mm negative/sensor size. Secondly, surely is the FL spec was given for a 1.6 crop sensor the angle would be wider for full frame rather than narrower? Can someone explain what's going on here?