The EF 300mm lens refers to a family of telephoto prime lenses made by Canon, five of which have been sold to the general public and one of which was only made on special order. 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.[1]