Put a Helios 44-2 on a Fuji body and it stops being a 58mm lens, at least as far as framing goes. It frames like an 87mm. On an Olympus Micro Four Thirds body it frames like a 116mm. Same glass, same aperture ring, very different lens in practice. Most of the confusion around adapted glass comes from mixing up three separate things: what the sensor crops, what the adapter has to do mechanically, and what a focal reducer changes.

Crop factor doesn’t actually change the physical f-number of a lens, but it absolutely changes how that f-number behaves in practice, and that’s where people get tripped up a bit. The f-number itself is purely optical. It’s defined as the focal length divided by the diameter of the entrance pupil. So if you mount a 50mm f/2 lens on a full-frame camera and then mount that exact same lens on an APS-C body, it is still a 50mm f/2 lens.