I don't think there is anything negative to say about the image quality of this lens. There are a few things a potential user should be aware of: the magnification range is only 2:1, which seems to be the standard for MF, and I suspect compromises or technical innovations from 3:1 (common in FF and APS-C) will add weight and bulk unless image quality is compromised. Fuji also has a stabilized GF45-100mm (also with 2:1 magnification and even more), so you need to be clear about which zoom range suits you best, or you might want both. The simpler question to ask yourself is whether 2, 3 or 4 prime (single focal length) lenses would perform better than limited range zooms. No image stabilization. However, this is not a problem for landscape photographers, who almost always use a tripod. (Personal handheld observation works better with the GFX50R than I expected. As far as I can see, ISO 1000 or 1200 can be used with ease. Also, the combined weight of the lens and camera seems to quell a bit of shake. Somewhat related to inertia?) You already know MF is expensive, so I won't comment on that. In my opinion there are NO design quirks, flaws, bugs or anything else to worry about - just whether it's fundamentally the right lens for a given user.
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