The Canon EF 100-400mm f/4.5-5.6 USM L IS is a very good performer with few weaknesses … that’s assuming you get a good sample – the first tested sample was mediocre whereas the second one performed very fine. The resolution of the lens is on a very high level throughout the zoom range with only a marginal performance penalty at 400mm. Vignetting, distortions and CAs aren’t really field relevant on an APS-C DSLR. Typical for Canon L grade lenses the build quality is great. However, you may need to get used to the push-pull zoom mechanism which is a bit unusual these days. Regarding the sum of its characteristics the EF 100-400L is THE still portable super-tele zoom for Canon users.
The Canon EF 100-400mm f/4.5-5.6 L IS USM Lens has an excellent focal length range that includes the longest focal length found in a Canon zoom lens. This long focal length and focal length range combine with very good image and build quality to make the 100-400 one of the most popular serious wildlife and sports lenses Canon sells. … Weighing 3 lb (1,380g) and measuring 3.6″ x 7.4″ (92.0 x 189.0)(DxL retracted), the 100-400 is solidly built and feels like it. One of my favorite lens features found on the 100-400 is image stabilization. … Considering the focal length range of this lens, the sharpness delivered is simply excellent. The wide focal length range and long max focal length make the Canon EF 100-400mm f/4.5-5.6 L IS USM Lens a highly versatile and a very popular lens. This is one of the lenses I most-frequently recommend.