The Canon EF 70-300mm f/4-5.6 IS USM was introduced in October 2005 to replaced the old Canon 75-300mm f/4-5.6 IS USM.
Although superficially, this lens looks very similar to the 75-300mm that it replaced, optically and functionally, this is a totally new lens, completely surpasses the old lens that it replaced. The build quality is similar to the old 75-300mm, in line with other Canon consumer lenses. It still uses a micro-USM focus motor rather than ring-USM, so full time manual focusing(FTM) is still not possible. However, the focusing speed is noticeable faster than the old 75-300mm, although still not as fast as other Canon zoom lenses with ring-USM. This lens has also been updated with the latest 3rd generation IS (image stabilizer), which can compensate for up to 3 stops of light. It also has a 2 position switch which allow the IS to compensate for movement in 1 plane or 2 planes, which is useful for shooting panning shots.
The optical construction of this lens is totally new with a UD(ultra-low dispersion) element. Thanks to this new optical design, the optical quality of this lens is light-years ahead of the old 75-300mm. Unlike most other zoom lens with this range, it is sharp with good resolution up to 300mm. Most of the other zooms with this range are soft at telephoto end.
The advantage of this lens is that it is fairly light. Combined with a good zoom range and good optical quality. This makes it a very good travel or hiking lens. I bought this lens in December 2006 and has become my default travel telephoto zoom because of these reasons. Of course when weight is not a problem, I would still prefer to use the Canon 70-200mm f/2.8L IS USM, which is a faster lens with larger constant maximum aperture and lightning fast ring-USM focusing speed.
The following are some sample shots I have made with this lens.