Im just getting into DSLR's and was wondering.... there is AF and AF-S... The AF-S lenses are alot more expensive. Why? What are the differences? I am getting a 18-70mm that comes with a D70 for normal shooting. I want another lens that has greater zoom for sport photos. I've been looking at a couple such as a 18-135mm, 70-300mm. The 18-135 has more zoom than the 70-300 but The 70-300 is also more, why is that?