We're having some frame rate problems on a pair of new 7Ds. One is about 3 months old and one is about 3 weeks old. They behave the same, so whatever it is must be "normal" or a setting mistake. And I have a hard time believing this is normal.
The problem is that the frame rate drops radically when the camera is pointed at a dim subject. Pat noticed it first trying to do flight shots in Texas, using fill flash at the time. We assumed we had the flash set up wrong and didn't try to diagnose it until we got home. I started with some non-flash tests at home and have weird repeatable behavior.
I can stand in a room that is part bright and part dim. Point the camera at a bright area and get full 8 fps. Point at a dim area and get half that or less. It seems to be related to absolute light levels, not difference from bright to dim.
The camera is in manual exposure, shutter speed high enough to not be an issue. Lens wide open, ISO normalish. It does not matter if the image is in focus or not, or if the lens is in AF or MF mode. We use back button focus. Capture raw only. ISO expansion on or off has no effect. Exposure safety shift on or off has no effect. Long exposure NR is off. High ISO NR is off. Highlight tone priority is off. AI servo tracking sensitivity fast or slow has no effect. AI servo 1st/2nd image priority is 2 - release/drive priority. Lens drive when AF impossible is off. AF assist beam is off. Resetting everything does not help.
I assume you mean the green dot at the bottom right in the viewfinder. It does not seem to be a helpful diagnostic. It is visible in One Shot focus with the lens on AF, but not with the lens on MF. It is also not visible in AI Servo AF which is where we leave the camera - we do back button focus. A bright subject is fast fps even if wildly out of focus, a dim subject is slow even if in focus.
Oh, and I should have said at first the the battery is over 90% full.
I was thinking about the indicator that tells you what point is in focus when you're in MF mode, perhaps using more cpu cycles trying to determine focus in low light?
It is a known 7D's limitation. Canon informs about this issue in 7D manual (bottom of the page 93):
- "In low-light areas or indoors, the continuous shooting speed may become slower even if a fast shutter speed is set."
It has something to do with a metering. There is a small trick around it but it doesn't work in all situations (like panning on a subject that is passing through different light) and it doesn't work in Manual mode. If you lock the exposure (AE lock doesn't have any effect in Manual exposure mode), 7D should get full 8 fps even in a dim scene (with a dim subject).
First, you need to set the release/focus priority mode correctly. In certain modes, priority is given to focusing, in this case frame rate will decrease if the camera found it difficult to focus. If you want the frame-rate to be maintained, you need to choose one out of the two release priority modes such that priority will be given to maintaining the shutter release instead of focusing.
There are 2 sub-modes for release priority, one is with tracking achieved, the other without taking care of the tracking, this later sub-mode should be the fastest as both tracking and focusing are given up for the boosting up of frame-rate (i.e. release priority without compromise).