Adapting M42 lenses to mirrorless: a practical guide
M42 is the friendliest vintage mount to adapt: the register distance is 45.5mm, longer than any mirrorless mount, so a simple hollow metal ring restores infinity focus. No glass in the adapter, ever — an adapter with optics is a teleconverter you didn't ask for.
Which adapter
Buy the boring mid-priced one with good reviews. The five-dollar ones are often fine too, but check for two things: a flange that doesn't let the lens over-rotate past the top mark, and machining clean enough not to shed aluminium dust into your camera. The hundred-dollar ones are nicer, not sharper.
The auto-aperture pin
M42 lenses stop down via a pin that the camera body used to press. On an adapter nothing presses it, so "auto" lenses stay wide open unless the lens has an Auto/Manual switch. Most Takumars have one; many Soviet lenses are manual-preset anyway. Learn your lens before blaming the adapter.
Focusing
Focus peaking plus magnification, in that order. Peaking lies at f/2 in low contrast; zoom in for anything that matters. And turn off "release without lens" restrictions in the camera menu — the camera doesn't know a lens is attached, because electrically it isn't.