The first thing I would ask is what brand of lens you need. If you have Canon or Nikon, long fast lenses made by these manufacturers are fairly easy to find in the used market, and you are talking a pretty hefty investment when you want long, fast glass.
I had a Canon 300 f2.8 manual focus...now have a 300 f.4 auto. I have used all the Canon fast lenses from 300 to 600 with various extenders, etc., and have found anything over 400 to be extremely limited for rr photography, for many of the reasons Ernest mentions. One of the reasons there are many Canon 300f2.8L autofocus lenses on the USED market these days is because many news and sports photogs have found it better to buy the pricey Canon 200mm f1.8, along with 1.4x and 2.0x extenders. Gives them fast focal length even with the extenders and the whole setup's easier to carry around. --Dave Busse -> SPORRS: Serious Photographers of Railroad Related Subjects -> Web Site: http://www.anet-stl.com/acphotog/sporrs
