If weight+size is your priority and if you need a f/4 aperture, then take the 400 DO. If quality is the priority and you do not care much about the weight, the 500 mm is the choice. lastly, if you can afford it, usually need a 300 mm and occasionnaly go beyond 300 mm, then 2.8 300 mm is what you need with the possible use of a 1.4x converter. http://www.photographyreview.com/mfr/canon/35mm-primes/PRD_104369_3111crx.aspx