I don't think there is any way to reassign (or lock) any of the function buttons during playback on any X series camera. 

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I'd love to know the answer to this too, so am bumping the thread. Thanks to anyone who can help Paul

Thank you so much, Jerryy! The article is very interesting and now I'm confident about the aperture stepping. About the parfocal property, I have information that the first FW releases didn't fix the initial issues (focusing mistakes in zooming), but a following resease solved the problem: I would like to have a confirmation by some actual user of the lens. Very kind of you, Jerryy!!

This review may be able to answer some questions: https://www.dpreview.com/news/3492623346/fujifilm-xf-18-120mm-f4-lm-pz-wr-lens-is-parfocal-weather-sealed-and-will-cost-just-899 The review seems oriented toward using the lens for video work. Note: If you are working outside of Europe, keep 1/60 in mind as a shutter speed. The line frequency is often 60 hz rather than 50hz.