This might be a silly question but it has been bothering me for a while. Say i have a D80 (although i have D40X) and ofcourse it has an internal autofocus. Considering there's 2 equal lens (max aperture and focal length), one is AF-S and one is AF-D. Which one is better? I guess my question is AF-D will be cheaper since it has no internal focusing so why would anyone pay extra for AF-S? does it have any advantages in the body that already has an internal auto focus? Thanks!