Some of you might be interested in this link for collimating the Canon 24-105L lens:
http://canonrepair.wordpress.com/201...5-is-usm-lens/
I just tried the collimating my own copy of the 24-105L using the technique described above (except using the star Canopus rather than a artificial light source) and the improvement was stunning! My lens is now a great performer wide open and at all focal lengths as a result. Attached is a comparision at the 105mm f4.0 - the weakest setting for this lens - showing before and after. Both shots are near best focus possible and are crops at 100% from a 50d.
Dissassembling this lens is not for the faint of heart and requires some patience - it's not for everyone!
Terry