I am a beginner with a D40 and I am looking to buy my first lens. I have the kit lens(18-55 f/5.6) and the 70-200(f/5.6). I'm looking for something in the 24-90mm. I have found a Sigma 28-70 and Nikkor 24-70, both f/2.8. My only questions are: 1)Will the Sigma 28-70mm f/2.8 EX DG AF autofocus on the D40? and 2)why is the AF-S Nikkor 24-70mm f/2.8G ED same range $1300 more than sigma's?