http://www.bhphotovideo.com/c/produc...0mm_f_1_8.html why is it so cheap? and what is the major difference between that and this one: http://www.bhphotovideo.com/c/produc...Autofocus.html and is 50mm the way to go if I HAVE to get a fixed range? I want the lowest possible aperture, and they are either fixed or $1,500. So unless some one wants to pay for me to get a $1,500 lens I have to get a fixed one. Help!