I have been lucky to be photographing whales for 25+ years, all without the use of IS. My 70-200 f4 on a canon 20D gave me enough reach along with higher ISO speeds for sharp images. Now I have a full frame 5Dmkii and decided to rent the 100-400 lens to have anough reach. I see there are 2 different IS modes Regular+Panning. I don't need to pan per se, but my question is should I use IS and would it make sense to just use in the regular mode. I am concerned because at f5.6 @ 400mm I don't want to push ISO too high. In addition, you need to be quick to catch the action as they break the surface, so would IS slow down the AF-to-shutter time too much?