It appears to me, that when i use the Mamiya Sekor SHIFT Z 75mm ƒ4.5 lens with the Fotodiox adapter on the Fujifilm GFX 50s II camera - that the horizontal area basically doubles. So, as if I would have a sensor with a length of around 83-84 mm.

The panorama was 100 MP, and it basically doubled in length. Yes, I could see some corner shading, because i shifted the lens to its extreme, beyond its normal 6x7 format, so to speak. It is as wide as i can make it. Which is more than i even bargained for.

Sharpness

I did have some issue with sharpness in the extreme left and right border, where the focus was a bit closer. I do not know if that is due to the lacking integrity (parallelism) of the adapter vs lens - or the lens has some minimal shift in the focus plane towards the borders. The focus enlargement in the Fujifilm camera isn't so good when you go into 10x enlarging mode. So, it should also been that i focused ever so slightly off (by slightly front focusing / e.g. focusing too close). Those very fine focus differences between let's say 35 meter and infinity distance, are minimal, often difficult to determine on the screen of the Fujifilm GFX 50s II.

But a little bit of gentle AI sharpness, did address the finest details, which then got chiseled out again. Not a problem as long the slight focus difference is minimal and not too blurry.

Wide open ƒ4.5 sharpness

When this lens is used wide open at ƒ4.5 - it reminded me of the more ordinary Pentax 6x7 Takumar lenses also when used wide open; some reddish fringing in the finer details visible. Where the fine branches of trees get a little bit reddish. Sharp but not super sharp. Once you stop down, it is sharper.

Lens Flare

I also noticed in the center image (of totally 3 images taken at aperture ƒ13), there was a large blob of stray light (from the sky) lowering the contrast and giving a lighter diffuse area in the shadows, below the middle center line. This did not happen in the shifted images (left vs right). I addressed this with local alteration of contrast and color (as the blob was a bit bluish). No problem.

How wide does the Mamiya SHIFT 75/4.5 get

I believe somewhere around... hm... 28 mm fullframe perhaps ? It isn't quite 24mm but much closer to 28mm, is my guess.

It means those 75mm turn into something more like a 28-30mm wideangle perspective when you shift 3 images and stitch them together. With superb fine details - which of course never will be fully revealed in the final image above. That image is equivalent to a 3.3 MP photo. The original is 100 MP.

Originally the Mamiya RZ 75mm shift lens is considered to be something like a 35mm lens in 6x7 format. On a Fujifilm GFX camera, it would become an 59mm equivalent lens (= a longish normal lens).

If I half this (due to that i use twice the sensor length of 44mm), then it turns into something like 29-30 mm lens = aka a moderate-ish wideangle lens, slightly wider than the view it shows on the original Mamiya RZ67 film camera in 6x7 format

So yeah.

Panorama equivalent to a 30 mm wideangle lens

I now tested it with the Olympus 12-40 mm zoom (24-80mm equivalent) just in order to see how the perspective corresponds to the main, panorama photo:

Result ?

Equivalent to a 29-30 mm wideangle (in my opinion), thanks to the Mamiya Shift function. Otherwise without shift - it is just a "boring" 60 mm longish-normal lens.

Well, now i know.