I'm looking to buy a telephoto lens for my Nikon D3100 and my budget is $500 max. Also, I'd like to get a lens that will allow me to Auto Focus. Any tips or recommendations would be helpful. I'm open to fixed or zoom, I just want to know what's best and my main desire is to have great depth of field. It was recommended that I get an 85mm f/1.8, but I just wanted to get more information and find out if this will Auto Focus on my d3100. Thanks in advance.