spanish_z said:
The 70-200 2.8 L IS has a 77mm filter. So I don't see why the 24-105 2.8 would need a larger filter?
The 16-35mm f/2.8L II has an 82mm filter size. OTOH, the MkI version of that lens has a 77mm filter - same focal length, same max aperture, yet a bigger filter on the MkII - why? 'Cuz that's what Canon decided to do.
BTW, for the mathematically inclined, 35/2.8 = 12.5mm, so clearly there's more involved than just focal length and aperture in determining the size of the front element. Presumably, there are also marketing and aesthetic decisions - the 100mm non-L macro uses a 58mm filter, and the 100mm L macro uses a 67mm filter, but the front elements are very similar in size, there's just a lot more 'dead space' around the front element on the L version.