The EF 300mm lens refers to five telephoto prime lenses made by Canon. The lenses have an EF type mount which fits the Canon EOS line of cameras.
When used on a digital EOS body with a field of view compensation factor of 1.3x, such as the Canon EOS-1D Mark IV, it provides a narrow field of view, equivalent to a 390 mm lens mounted on a 35mm frame body. With a 1.6x body such as the Canon EOS 650D, it provides a narrower field of view, equivalent to a 480 mm lens mounted on a 35mm frame body.
This lens is most commonly used by sports and wildlife photographers, but is short enough to be use for extreme close-up portraits. Because of its rather high native magnification, it can be used for some macro type photography.
Canon EF 300mm lens Wikipedia
The EF 300mm is a L series lens. This lens is constructed with a metal body and mount, and with plastic extremities and switches. Features of this lens are: a wide rubber focus ring that is damped, a distance window, the ability to limit the focus range, and an image stabilizer (on the IS versions). The maximum aperture of f/2.8 or f/4 gives this lens the ability to create shallow depth of field effects, while the eight- or nine-blade diaphragm produces smooth background blur. The optical construction of this lens contains 15 lens elements, including two UD (Ultra low dispersion) lens elements for the f4 IS USM (for other configurations see chart). This lens uses an inner focusing system, powered by a ring type USM motor. The front of the lens does not rotate nor extend when focusing. This lens is compatible with the Canon Extender EF teleconverters.