The Canon 100-300mm f/4.5-5.6 IS USM telephoto zoom lens was released in September 1998. The lens uses an older ''push-pull'' zoom design that extends the lens from its length of 189mm (7.4'') to almost 260mm (10.2'') when set to its most telephoto setting of 400mm. Incorporating both image stabilization and an ultra-sonic motor into the lens makes for a very compact and efficient package, weighing in at just 1,380g (just over 3 pounds).
The lens is fully compatible with full-frame sensors and film bodies, so there are no issues with vignetting. Canon digital SLR cameras with sub-frame sensors have a ''crop factor'' of 1.6x. Thus, for this particular lens, it will exhibit an effective focal length of 160mm to a whopping 640mm. It can also mount 1.4x and 2x extenders, meaning that you could see a potential of 1,280mm, if you're amenable to a maximum aperture of f/11 at that focal length. Read the whole review here. You can order the lens here