With a maximum angle of view of 104º, this lens uses a large aspherical lens for element 1 and another aspherical lens for element 15. They correct distortion and greatly improve peripheral resolution. The lens also accepts screw-on filters (77mm).
Lens Number UM0201

http://www.photozone.de/Reviews/176-canon-ef-17-35mm-f28-usm-
l-lab-test-report--review

ALso several filters from soft focus to UV etc

The build quality of this lens is very good with smooth but not overly well damped control rings. The outer length of the lens remains constant though the inner lens tube moves a little when zooming (if you look closely you may see this characteristic in the two product images above). The front element does not rotate so using a polarizer remains easily possible. The lens has a ring-type USM drive based on a front-focusing system resulting in an extremely fast AF speed. Typical for ring-type USM lenses full-time manual focusing remains possible in one-shot AF mode.