Hi Russell. Thanks for having a go at this. It is a really tricky problem.
The software has a locking button, where you can lock the shutter speed and ISO. It controls the camera operations. Even when I activate this, the image still seems to waver around - I can see the histogram moving up and down and from side to side, indicating varying exposure.
No one so far has been able to answer the question of whether it is to be expected that the histogram should move around - even when all other factors are stable, i.e. static subject, steady light etc. My instinct says it shouldn't, or very little. Canon looked at the camera, but didn't seem to think there was a problem...
I have looked at the original shot, and the control panel does not preserve the original settings. I have looked in the metadata, but this does not help much either. I have also re-read the manual, and this does not indicate any auto-adjustment control. The software is the industry standard for this sort of thing, so is intended to keep your exposures consistent.
I keep coming back to the question of whether the camera should be capable of taking a series of stills at exactly the same exposure levels, or whether it is considered normal for it to have a certain 'fudge factor' which allows it to hunt around? (I have tried taking a series of stills in this way, unconnected to the computer, and still come out with differences from one exposure to the next.) This doesn't seem right to me.
I noticed that on the camera I cannot operate the Exposure Lock when it is in Manual mode. I assume this is normal. But the software manufacturer tells me that the camera must be linked to the computer in manual mode, via Live View, suggesting to me that it might not be able to lock the exposure, if I can't.... In which case, should I be able (i.e. is this intended by Canon) to lock the exposure in manual mode, and would this indicate a fault in the camera? (Of course this last point is irrelevant if the Exposure Lock does not continue from one shot to the next.)