Adapted Canon 70-200 IS manual focus assist issue

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Adapted Canon 70-200 IS manual focus assist issue

I'm hoping that somebody can help me with what is a bizarre phenomenon.

Setup: Canon 70-200 F4 IS, Sigma MC-11 adapter, A7III. Commlite adapter tried too.

After noticing some manual focusing issues I set up my camera on a tripod a few meters away from some wallpaper to check manual focus. The camera is absolutely perpendicular with the wall, and a timer is being used.

Using the rear screen I was surprised to discover that I was unable to achieve sharp focus using the magnifier - irrespective of how carefully I adjusted focus. The image would pass through the focal plane without ever being sharp - very mushy in fact. Just to be clear, this is before a photo is taken.

However, as soon as I moved the focus magnifier rectangle away from the centre of the frame, towards the bottom, it was very easy to achieve sharp focus with the magnifier.

What's strange is that on taking a photo (with bottom in tack focus according to the magnifier) the resulting image shows that the centre is in fact extremely sharp, whereas the lower part of the image is slightly out of focus.

If I were using a DSLR I may have attributed some of this to focus shift - but this isn't the case. The aperture isn't being stopped down when I take the photo (it's the same problem if I test wide-open), and the camera isn't moving at all.

Plus, even if focus-shift was the cause, it wouldn't explain how it's impossible to achieve sharp focus using the magnifier in the middle of the image.

I'm very familiar with the limitations of the magnifier, but this doesn't explain why it is only capable of providing a tack-sharp preview away from the centre of the image.

This isn't my first experience with adapting lenses, but it's the first time I've experienced anything like this.

Any suggestions? Thanks.

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