Quoted By:
The real reason old SLR lenses work on mirrorless is because flange registers 18mm or smaller make it possible to engineer adapters that mount the lens at the appropriate distance from the sensor, even for rangefinder systems like L39. E-mount happens to have been around for over a decade, so it's well supported, but Z-mount might get there soon enough if Nikon eats Sony's lunch by making cameras that people actually like to use.
The one thing to note is that the focusing scales on old lenses seem to be based on a circle of confusion near 0.03mm, which is barely workable with APS-C in my experience (pic being f/16 hyperfocal with a Vivitar 28mm f/2.8). Full-frame will be more forgiving if you want to use the manual focus features like they were designed for.
