Quote Originally Posted by neuroanatomist View Post
"Fast" is relative, of course. But Canon makes lenses with an image circle sized for a 2/3" sensor that achieve 930mm f/4.7 (that's real focal length, not 'crop factor adjusted' which would be 3655mm f/18). These are field lenses for broadcast HDTV - for example, the DIGISUPER 100 xs is a 9.3-930mm f/1.7-4.7 lens, with a built-in 2x TC for 1860mm f/9.4, and it's an image-stabilized lens.
Not only is it image-stabilized, there are internal weights that move around "opposite" to the zoom/focus actions, so that the balance of the overall rig doesn't change during zoom and/or focus. Let's hope Canon doesn't do that to our 100-400s or 28-300s.