The problem has been solved, see post #12.
I have upgraded the firmware from 1.3.1 to 1.4.0 and voila! The problem is here... Canon has broken something in light metering in still photos mode.
I'm using my R5 on the telescope (no aperture and focus info) for contrails spotting, like this one for example (taken about two weeks before upgrading the firmware):

But after upgrading to 1.4.0 I'm not able to do that any more! And what's worse - I'm not able to downgrade the firmware to 1.3.1 because Canon doesn't want me to do that... what a mess.
The problem is somewhere in the firmware definitely:
on the screen or EVF we have our picture with exposure simulation, the histogram shows everything OK, but after taking the shot, the picture is completely over exposed. When I put the under expose correction (for example full -3EV) to move a histogram completly to the left, after the shot it is completly to the right side - it is impossible to take a good exposure shot.
The problem is only in still pictures mode, in the movie mode everything is OK.
Also there is no problem in still pictures mode when using RF lens.
So the inconsistent exposure is only with let's say - third party optics, like my telescope. If it was mechanical failure of metering system, it would be in all modes and scenarios - here it isn't.
I've tried to reinstall the 1.4.0 firmware - no change (yes, you can reinstall the same version of FV, but you cannot downgrade it to the last known working).
I've switched beetwen all the still modes available (P, M, A, S and so on) - no change.
I've switched beetwenn CF and SD cards - no change.
I've switched beetwen RAW, JPG, DualPixel RAW - no change.
I've reseted all the camera settings - no change.
The problem is even in full Manual mode - I'm chosing the correct settings for exposure, and the picture is still very over exposed.
I'm talking of about 15-20 EV stops! Full spectrum of histogram from left to right!
I really don't have any more ideas to find the solution.
CANON - please repair it immediately! And give - us - photographers - the choice to downgrade the firmware when you broke something in the new version!
By the way - there is also some issue in timelapse mode with electronic shutter - camera is taking 2 - 3 shots, instead of one. But this bug is for some other topic.
I have upgraded the firmware from 1.3.1 to 1.4.0 and voila! The problem is here... Canon has broken something in light metering in still photos mode.
I'm using my R5 on the telescope (no aperture and focus info) for contrails spotting, like this one for example (taken about two weeks before upgrading the firmware):

But after upgrading to 1.4.0 I'm not able to do that any more! And what's worse - I'm not able to downgrade the firmware to 1.3.1 because Canon doesn't want me to do that... what a mess.
The problem is somewhere in the firmware definitely:
on the screen or EVF we have our picture with exposure simulation, the histogram shows everything OK, but after taking the shot, the picture is completely over exposed. When I put the under expose correction (for example full -3EV) to move a histogram completly to the left, after the shot it is completly to the right side - it is impossible to take a good exposure shot.
The problem is only in still pictures mode, in the movie mode everything is OK.
Also there is no problem in still pictures mode when using RF lens.
So the inconsistent exposure is only with let's say - third party optics, like my telescope. If it was mechanical failure of metering system, it would be in all modes and scenarios - here it isn't.
I've tried to reinstall the 1.4.0 firmware - no change (yes, you can reinstall the same version of FV, but you cannot downgrade it to the last known working).
I've switched beetwen all the still modes available (P, M, A, S and so on) - no change.
I've switched beetwenn CF and SD cards - no change.
I've switched beetwen RAW, JPG, DualPixel RAW - no change.
I've reseted all the camera settings - no change.
The problem is even in full Manual mode - I'm chosing the correct settings for exposure, and the picture is still very over exposed.
I'm talking of about 15-20 EV stops! Full spectrum of histogram from left to right!
I really don't have any more ideas to find the solution.
CANON - please repair it immediately! And give - us - photographers - the choice to downgrade the firmware when you broke something in the new version!
By the way - there is also some issue in timelapse mode with electronic shutter - camera is taking 2 - 3 shots, instead of one. But this bug is for some other topic.
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