First, I downloaded XNote Stopwatch which is a Windows application that will display the time in hundreds of a second running on a laptop. The Canon 7D is mounted on a tripod with an 18-200 lens zoomed at full telephoto with max aperture of F/5.6. An ND4 filter is attached to cut the light even more. I shot three sequences, all with manual exposure and manual focus ... so the camera (standard settings with JPEG only) should just rip through the frames, not worrying about anything.
I turned the laptop brightness all the way down and at F/5.6 & ISO 6400, the image was still underexposed at 1/100 second, but I didn't want to go any slower for burst shooting. The Canon 7D would consistently only shoot four frames/second!
If I cranked the laptop brightness back up, (but changed nothing on the camera), it would then shoot eight frames/second. For grins, I shot a third sequence with the ND filter removed at ISO 1600 and 1/200s ... it squeezed in one more shot, but probably more due to the faster shutter speed.
I did the same tests with the Canon 50D, and no matter what the light
conditions, it would shoot at 6.5 frames/second when in full manual,
even when shooting in a pitch black room. I consider this an
interesting oddity of the camera since I'd rarely "machine gun"
in light as low as F/5.6, ISO 6400, and 1/100s (which underexposed), but
there does seem to be something
different about the Canon 7D versus the 50D.
Assuming the metering is the underlying issue, an easy firmware fix
would be to stop the "meter check" when in full manual after
the first frame of a burst ... since you really only need it before shooting.
The Canon 7D images are below with EXIF. You can also look at
the shot times (they both record to a 1/100 second), and the summary data is:
13 shots in 2.90 seconds - Sequence 1 demonstrating slowdown
23 shots in 2.98 seconds - close to rated 8fps, even with 1/100s shutter speed
24 shots in 3.01 seconds - shutter speed bumped to 1/200s
Note: You can also simply shoot in a pitch black room to see this behavior! ;-)
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