He also was referencing metabones, which does change it.
Anyway -- a teleconverter works by spreading the image circle out to cover more area, right? It's not actually the same as making the lens zoom twice what it previously was, that's why it's darker. Right?
A 2x teleconverter doubles the size of the image circle, so a crop lens on a crop body would be effectively doubled.
A full frame lens on a crop body would be doubled, too, of course, but only the part that touches the sensor matters.
A crop lens on a full frame body, though… that would typically cover, what, 2/3 of the sensor/film? So if you used a 1.5x teleconverter, that would spread it approximately enough to cover the film plane without losing much, which would make a crop sensor 50mm give the same 75mm field of view on the full frame that it would have given without the teleconverter on a crop camera, right?
and why am I thinking about this stuff when I'm supposed to be coding? Work ethic fail.