That's not entirely correct.
While the 120mm doesn't go to 1x on its own and is quite abysmal with extension tubes, it's an excellent lens at its native magnification of 0.5x
Yes, JK has proven beyond reasonable doubt that for flat field/ copy work the GF120 with 45mm tube is not the sharpest crayon in the box. But, abysmal? I would say that's a gross overstatement.
View attachment 728094ae7e514eada1e73816b45ff90d.jpg
It's not an overstatement. It's accurate.
https://www.dpreview.com/forums/post/67180601
The effect gets worse the more tubes you use (and the higher the magnification gets).
Even in the example you posted here the image quality outside the center portion is really, really bad. More pixels don't help, the distortions, aberrations, smears, coma can't be fixed with more "data".
I've sampled a lot of really affordable alternative lenses that deliver a much better image quality at magnifications higher than 0.5x - and those deliver pretty much the same high image quality over the whole image, not just the center portion.
I wouldn't use the 120mm with tubes, that's just a waste of image quality and doesn't do the lens (or the sensor) any justice. Instead it would be a better idea to just crop the image, even a crop of an image taken with a 50 MP sensor at 0.5x would be preferable to a 100MP image taken at 1x.
Here's a comparison between the APO-Rodagon D 75mm 4.0 (1:1) and the GF 120mm with tubes:
APO Rodagon 75mm left / GF120mm with tubes right.
So you can see that while the GF120mm is an amazing lens when left alone, it's... well... a glorified ashtray when used with extension tubes.
EDIT: besides, the native Fuji-Tubes are complete and utter rubbish. They're too narrow, resulting in vignetting when using multiple tubes. They aren't flocked so you'll get in trouble with internal reflections too. So avoid them at all cost.
The best tubes I've found so far are the ones made by Fotodiox, they're totally flocked, have a wide throat with 0 vignetting and just as solidly made. They're not the cheapest tubes around but still far cheaper than the Fuji tubes.
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Professional naysayer. Occasionally.