The ef17 40mm f4l usm is a lightweight ultra wide angle canon ef zoom lens it is especially well suited for use with digital eos slr cameras with a focal length conversion factor higher than 1 superb optical performance throughout the zoom ran fastest af speed within its class highly dust and moisture resistant 28cm minimum focusing distance at all focal length ef mount l series lens aperture range f4 22 three aspherical elements one super ud ultrasonic a