Ok new 7d owners, I want you to try something out here. I have confirmed that this happens on at least one other 7d apart from my own. Once you have tried it, does anyone have any idea WHY?? it does it.
So...put the camera in high continuous mode. ONE SHOT FOCUS Put it in TV mode and set a reasonably fast shutter speed so that you can get 8fps. Pull the trigger and let 'er fly. It will rattle along at 8fps. Now put your hand in front of the lens or swing it round to point at a dark area in your house or something. All of a sudden the shooting speed drops!
Now this is in one shot focus so there is no focusing going on to slow it down. This had me puzzled. So i tried it in manual exposure mode. Same thing!! In manual mode, the camera should surely just rattle off the frames without thinking, doesnt matter if they are correctly exposed?!?
So i tried a few different settings and found out that you can stop this by using the AE lock button ( or setting AE lock to the shutter button)
So this would indicate that something in the exposure system is causing the slowdown. Once locked to the exposure using the AE button it works. In which case why does it do it in Manual mode?? In manual mode the AE lock button does not function so there seems to be no way of preventing it happening in M.
It also does the same thing if you switch the auto focus OFF on your lens. Seemingly ruling any weird focus things out.
I looked in the manual. Indeed there is a note at the bottom of the page about continuous shooting stating that the speed may vary in dark rooms. But it offers no explanation, and I dont remember this being a factor in any of my previous cameras. I just tried my 5dII and it certainly doesnt behave in the same way.
I know the likelihood of this ever making any difference to me is slim but I just wanted to know why id does it??
It also varies depending which lens you use. Sticking an f4 lens on it makes it considerably more susceptible. Even when shot at equivalent apertures. I put my 50mm 1.8 on the camera and aimed it at a dark corner of my room but set the aperture to f4 and achieved a much faster shooting rate than i did with my 24-105 f4 even though i was shooting in MANUAL exposure mode.
EDIT:: See my post below. It is NOT something to do with Custom Functions as far as i can tell.