When adapting manual lenses to the Canon RF mount, there’s an issue with the image not matching the exposure settings and shown in the viewfinder.

Using a simple ring adapter (with sensors) converting m42 lenses to EF mount and fitting it to the Canon EF to RF mount adapter, results in an image that appears fine in the viewfinder, but over exposes the image taken (using the exposure settings in the viewfinder).
Dropping the exposure down 4 stops, exposes the image correctly.
However, this creates another problem.
Now the image in the screen and the viewfinder becomes very dark, to the point where you cannot really compose it.
To compensate, I could drop the ISO (on auto with manual mode), but this would involve several test shots before I could start taking photographs and if the light changes when I’m out and about, would need some more test shots and could result in my missing some shots.

*Sigh*

So I checked all the settings on the Helios lens – OK
I checked the viewfinder exposure menu option – set to “Enable” which exposes the viewfinder to ambient light – set

This puzzled me.
When I use a basic ring adapter (without any sensors on it) to convert from Olympus to EF mount, then fit it on my RF mount adapter from Canon, I don’t have this issue.
As I adjust in the viewfinder (manual settings) then take the shot, image matches the viewfinder for exposure.

*Sigh*

Was I doing something different with the two lenses?
No.
Was anything different between the two lenses?
Yes.

The Olympus lens (basic plain adapter) has no focus peaking available.
The Helios lens (basic adapter with sensors on ) has focus peaking available on screen, but has the issue with exposure of shot not matching viewfinder camera settings exposure.

*Sigh*

AHA!
So I tried my old 50mm Chinon lens, which has a simple Chinon to EF mount adaptor, with sensor pads on it.
I again can use focus peaking, but the exposure of the shot doesn’t match the settings or viewfinder exposure.
(Taken when exposure line moved to the middle of scale, the picture is blown out white. Taken when exposure is dropped 4 stops on the exposure scale in the viewfinder, image is correctly exposed, but hard to compose as viewfinder/screen is so dark).

So the solution is…
Basic ring adapter – without sensor pads – will correctly expose using viewfinder/screen exposure scale BUT focus peaking will not be available.
Ring adapter – with sensor pads – will allow focus peaking option, BUT, almost impossible to compose image while requiring 4 stops less of light (according to exposure scale on viewfinder/screen.

As I use zone focusing with manual lenses, the shutter speed is vital to prevent blur for anything moving into the frame, so the adapter that exposes correctly with the Canon R6, is the only way to go.

I find it ludicrous that in order to use Canon’s focus peaking system with manual lenses, you have to reduce the light by four stops (if using a standard adapter with sensor pads on it), to the point where it’s almost impossible to compose a shot (too dark, image exposure doesn’t match the viewfinder) or failing that, play about with ISO on the hoof, (which defeats the object of having auto ISO available) when in manual mode.

I hope this helps somebody, as it took over an hour to figure it out.

Be well.