I'm photographing sports with R5 and RF 70-200. I start at M Mode, SS is 1/1000 and aperture at f/3.2, the ISO floats because it's at "Auto". Later I peek at the settings, and the shutter speed is not where I left it, but has changed to 1/640, or 1/400, but it's always less than the original settings. This has happened in the FV mode also. What can a photographer do that could cause settings to change without using the dials?
From my own experience the algorithm seems to work like this...
If you set the ISO to variable, it has maximum values set by default or by the user, depending upon the camera model - the R5 certainly has controls for this. If the ISO reaches a maximum value, and the image is still under-exposed, it will not exceed that max ISO, but will then change one of the other variables and in this case the most likely candidate seems to be shutter speed. In your case at 1/1000sec., the shutter speed is well within the capabilities of the camera to decrease, but at f/3.2, the aperture does not have much potential to change (I am assuming you are using the f/2.8 version of the RF 70-200).
So, it might be worth checking what the ranges for auto ISO are for your camera.
For the EOS R5, it might be worth while to look at "safety shift" in the settings. Maybe if "safety shift" is set to shutter speed instead of ISO, the shutter speed will be changed to change exposure. But, the manual does not say that it does anything in "M" mode.
Thank you for the safety shift idea. I checked mine and my safety shift is like your image, "off".
During today's shooting, I observed the shutter speed change behavior only once. I'm photographing sports and there's a prolonged stoppage of play, about ten minutes or so. I start editing images to delete while in preview. The game begins again, so I press the shutter and presto, the shutter speed is down to 1/640 from my previous setting of 1/1000. Because I'm looking for it, I noticed the change and corrected the setting.
To Waddizzle, my FV mode uses the control ring. While using M mode, I don't have it set to do anything. I find the control ring a little awkward, but it's useful.
Thank you all for your suggestions. For now, I'm going to pay more attention to the settings because I know they'll move around on me. And tape gets in the way of the other controls.
Set the camera to M mode.
I would also disable the ISO Auto for a few test shots. Leave on Auto, see where it dials in a an ISO Setting, and then dial in that ISO Setting Manually. Does the SS still change?