I've tried using extension tubes on mine. Autofocus becomes unusable on the R5, but I can manually focus *IF* the subject is slow enough. Unlike many lenses the extension tubes don't accentuate chromatic aberrations very noticeably100-400 IS II + 2X II + 2x III + 1.4x III
Focus was hard, it would break in the middle of the zoom range, but focus at 400mm?? not sure what was the issue, but manual and auto focus both were hard.
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