My local camera store is having a clearance sale and they have a Sigma 17-35mm f/2.8-4. After reading some reviews, it's been hit for being soft in the corners and having a fair bit of distortion a 17mm. I really want a wide zoom of some sort, with a 50 prime and a tele-zoom. Is this a better choice than a Nikon kit lens at $100, if I could get this lens for $175? And does it have auto-focus on a D40? It says it has an HSM motor, is that anything like AF-S or AF-I? Thanks!