Originally Posted by
rpcrowe
I don't have the 100-400mm f/4.5-5.6L IS lens. I have the Mark II version of the lens.
The original 100-400L Mark I is a very well respected lens for wildlife and was a standard tool for safari photography for many years. It will certainly give you more reach than your 55-250mm lens.
One caveat is to experiment with your new lens at various focal lenghs to determine the slowest shutter speed that you can use and still get sharp images. Do that before you go out and shoot anything really important.
A monopod might be a valuable addition to your kit and help you get sharper images at slower shutter speeds. Don't forget, when you have your lens cranked out to maximum focal length, you will be shooting at an equivalent 640mm.
If you do get a monopod, by all means leave your Image Stabilization selected. I am not sure, but it might be a good idea to turn off the IS if you are mounting your package on a tripod...
When I hand-hold my 100-400 Mk.II, I most often shoot at ISO 320 at the minimum (and quite often higher). Most cameras will provide decent image quality at that ISO and it will help boost your shutter speed into a usable range. However, if push comes to shove, I'd rather have a sharp image with some noise than a noise-free image that is not sharp due to camera shake.