@Gargron A that 's the trick with aperture. And M42 lenses are a good wild card to be adapted to nearly every lens. I've bought two M42 lenses together with an old Practika recently. One of them has kind of an A-M Automatic-Manual switch to close the aperture.
@Gargron I heard something similar. My father had a Fuji he could use his old M42 lenses with an adapter, but only in manual mode. The aperture had to be closed with test button on the camera.
@macke_47 M42 to FD, and as far as I'm aware has no effect on aperture. I've shot with f2 through it.