The 120 ASPH is magnificent and I am not sure many have one and can really comment on it. I have one in Alpa mount. It is sharp at infinity edge to edge with an achromatic 150 megapixel back. The 138 is a cut above, yes, but this is fantastic just beneath this. 100% zoom crop, scaling is off here on the forum and depending on the screen / browser settings, but I can tell you that on a pixel level it is SHARP.
It is very compact, has endless shift and is sharp with low distortion. It is a no-brainer as are all the other next gen SK lenses, incl. 28 XL, 43 XL, 60 XL...
The big version defeats a large part of its allure: the compactness vs performance ratio on a tech cam. The one sold on ebay unfortunately didn't have a helicoid so a bit of a different use case, ie Actus, F-metric, etc. rather than Alpa / Cambo, etc.
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Below a strip of the whole scene; the first crop is a tiny rectangle from the left, the second one a 900 pixel wide crop from the right hand side.
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This lens pops up only every few years and I can only iterate that whenever you see a next-gen SK lens you should pick it up as prices only can go up over time given out of production status and excellence.