Hey everyone, I'm having a sort of strange problem with my Canon 300mm F4L IS and 1D Mark III, I'll try to articulate it as best I can. Earlier today I was shooting NCAA Tennis Finals and everything was going swimmingly (the lens was sharp, autofocus was fast, etc.) until I tried to stop down from f4 to anything smaller (f4.5, f5, f5.6, etc.). With the lens at f4 in AI Servo mode (continuous AF) I was able to make use of my body's 10fps, but when I stopped down the lens, without changing any other settings, my framerate dropped noticeably from 10fps to probably around 6-7fps. I do the vast majority of my shooting in manual mode and am confused by what caused this - I tried increasing my shutter speed and ISO, but nothing seemed to get my FPS back up to 10 apart from opening the lens back up to f4. I don't have this problem with any other of my lenses on this body, and tested to make sure by immediately swapping the 300 f4 for my 70-200 f2.8, which achieved 10fps at f5.6 (as it should). And no - I did not have the IS on the 300 f4 turned on! (Lens was set to AF, IS off, 1.5mm minimum focusing distance while 1D was set to 1/1600, f4.5, ISO 640, continuous high speed motor drive). Has anyone ever seen anything like this before? Or am I just being a complete idiot on a public message board by overlooking something obvious? I'm hoping this is a "doh" moment.