Hi all. I notice a news story about Metabones having produced a focal reducer for F-mount lenses to the Pentax Q. This is of precisely no interest to me (or almost anyone else), but there's a comment that it's "compatible with all lenses except the 2.1cm f/4 and 20mm f/2.8 AI-s", which confuses me. Okay, the 2.1cm f/2 has an invasive rear element (and I'd put money on some old fish-eyes not mounting either), but I've not heard of anything odd about the 20mm f/2.8 AI-s which would make it unusually incompatible, unless I'm missing something. This kind of thing normally talks about F3AF lenses (not that there's AF) and the medical Nikkor, some old shift lenses and ones using the focus unit, but I don't normally see the 20mm in the list. Is it a typo, or is there something obscure I can learn? In mildly related news, I'd only recently noticed the D3400 is missing the switch that engages the aperture ring driver, which I assume is why it can't work with pre-G lenses. I'm a little surprised you can't override that (can you?), given that a non-G lens should work just fine if set to minimum aperture; Nikon seem to have put idiot proofing ahead of compatibility, admittedly on their most idiot-friendly DSLR. Another year, another "so much for compatibility with everything from the 1960s" F mount change.