The ef17 40mm f4l usm is an ultra wide angle canon ef zoom lens meant for professional and high amateur slr eos camera users while it is an excellent lens when used with an analog slr camera the ef17 40mm f4l usm is especially well suited for digital slr cameras with an imaging sensor size smaller than the 35mm format indeed on such digital slr cameras these focal lengths can give an angle of view approximately equivalent to that of a 28 70mm lens used on a 35mm camera which makes it th