Bought a 100-400 f/4.5-5.6 L last year and didn't use it much because winter set it shortly after buying it. For those of you who do bird photography what do you consider the minimum focal length needed for the job? Since buying the lens I purchased a heavy ball head, tripod and a gimbal that attaches to the ball head. During the winter months when I wasn't to active I purchased the 1.4 and 2.0x Canon tele extenders. Didn't know at the time that I need a f/8 capable body to use them. They do work with my ancient 1Ds but I want the extra reach of 1.6. My dSLR's include the 1Ds, 6D and 70D.