The Canon 100-400mm f4.5-5.6 is a super telephoto L series zoom introduced in 1998 that is compatible with both full frame or APS-C sized sensor Canon cameras. When used with APS-C cameras such as a Canon 7D, there is a crop factor of 1.6 so when at 400mm it is the equivalent of 640mm. When paired with the 1.4x and 2x extenders, you can reach considerable focal lengths but expect to lose some sharpness and light.
Just like the most popular Canon telephoto, the 70-200, the 100-400 is solidly built but features a push-pull design with a ring that controls the tension of the zoom. This lens also features a dual IS, image stabilization, mode. Mode 1 stabilizes both vertically and horizontally, Mode 2 only vertically. This means that if you are shooting a moving subject and need to pan, you would shoot in mode 2.
When it comes to sharpness you have to expect the lens to perform better closer to 100mm than 400mm. In the 100mm to 150mm range to get the sharpest image it is best to shoot at f8 and when fully extended at 400mm f11 to f16 will give you the best results.
When shooting on a full frame sensor camera, its important to keep in mind that there might be a little vignetting, especially when opening the iris to more than f8. At higher f-stops, the vignetting is very small, less than a quarter of a stop. When used on APS-C sensor cameras, even at lower f-stops, vignetting is very hard to detect, less than a quarter of a stop.
As the only Canon zoom that reaches 400mm, the Canon 100-400mm has excellent focal length range, sharpness, great image quality with little chromatic aberration, no distortion and great color contrast, making it the perfect lens for landscape, wildlife or sports photography