HunterW wrote in post #14743310
thanks for your guys input it very interesting and exactly what i wanted to know!
i have one more semi related question
is low light AF performance the lens or the camera or both?
Both. The 135L is one of Canon's fastest-focusing lenses, and will be fast regardless of the camera, but mounting it on a 1-series or 5D3 will definitely show improvement. Counter-wise, the 85L is quite slow, and mounting it on a camera with a top-shelf AF system like the 1Dx or 5D3 will improve accuracy, but not necessarily speed.
Most AF sensors typically lock off their cross points to lenses slower than f/2.8, but this isn't always a detriment as the 70-200 f/4 IS is still one of the fastest-focusing lenses Canon makes. The new f/2.8 IS II lens may be a hair faster, but you're counting the RPMs on F1 race cars at this point.
Returning the the previous topic a bit, one occasionally over-looked point of having a "better" lens is the maximum aperture, because sometimes money simply buys you better physics. It's one thing to simply have a lens that goes really wide open but looks like crap, and another thing if the lens can maintain a fast aperture and still render a razor-sharp image. The 24-70 f/2.8 II and 70-200 f/2.8 IS II are both super-pricy lenses, but changing the aperture on them should only be considered for depth-of-field; as they will be completely free of any aberrations, loss of sharpness, and so on, even at f/2.8.