It is not a flawless experience when adapting the Canon EF 135/2 L to the Fujifilm GFX camera (via Fringer EF-GFX adapter).

Hard vignetting in only extreme corners

It does hard vignette when you stop the lens down. It is that simple. But just to be clear - it is a rather minor hard vignetting which can be erased in post processing.

Overall, it seems that the Sigma ART 135/1.8 is the lens which most users who have used and tested this lens - is optically and overall best as an alternative GFX lens. It covers the entire Fujifilm GFX sensor. Apparently also does the the manual Samyang 135/2 lens.

Also:

When you have straight lines in the photo, you'll see in the originals that there is a slight bending of lines. This I attribute to the thick sensor glass in the GFX. The Canon EF isn't optimized to work on a Fuji GFX camera nor covering the larger image circle of that sensor. So, going Third party always means some things can be off.

Sharpness appears to be top notch, with very fine micro details - especially stopped down. (In the image above I used the aperture ƒ11) Yet, the depth of sharpness is still minimal...

I like the lens for portraits, with large apertures.