About the lens
The Canon EF 100-400mm F/4-5.6 IS USM II is the updated version of the popular Version 1. They have gone from a pump system for the zoom to a regular twist one. A great upgrade as the version had a tendency to suck up dust.
It’s one of my favourite lenses for photographing medium-sized to large sized animals. The zoom range is great both on full frame and APS-C and the versatility of a zoom mean you are less likely to miss a shot if the animals come in closer. When you are on trips where you can’t know the distance to the animals (i.e boats - safaris) and rely on a range of parameters to be placed in the perfect shot position, a zoom is the safest option to get a shot you will be happy with. The range on an APS-C camera is really good (eq: 160 – 640mm!) and it’s sharp at all focal length (no I haven’t gotten an MTF chart out but in practical use and on the field I have never looked at an image at 400mm and thought “gee that is soft”. I prefer rating gear like that rather than in a studio where the wildlife doesn’t move very much!). It balances well on most Canon XXD and XD series DSLR while allowing for extended handheld periods to be rather painless. It’s weather sealed, another great feature for the wildlife photographer as you, generally, have to be outside to find wildlife!