I will eventually be wanting to buy a zoom lens for my D70S. This is my first SLR so I am very confused on a lot of this stuff. I have been looking at lenses and they are all measured in mm's. Having no experience with this sort of thing though the numbers mean very little to me. I know that a 200mm zoom will not get me as close as a 300mm, but that is about all I know. Is there some way of knowing (other than actually looking through the lens) how much magnification I get from a particular focal length?