Originally posted by Lowell Goudge Note, that the pentax M42 mount in its final generation did require precise rotational orientation, specifically for the Spotmatic F and ES II which had open aperture metering achieved by the same aperture reading lever as the origonal K mount.
Yes, but lacking the locking notch/peg they (S-M-C lenses) required a separate tab to precisely index the lens relative to the body.
Quote: As for the precision of the flanged adaptor, its locking is just as accurate as any pentax lens.
I'd say secure rather than absolutely precise/accurate. Where an M42 thread mount (of any brand or quality) rotationally indexes on the body is determined by where the lens thread "terminates" as it's screwed into the body. Controlling that requires either an uncommonly high degree of precision in manufacturing or additional, adjustable barrel/sleeve parts, assembly/calibration steps and QA.
I've "shaved" threads on M42 adapters to index the aperture ring left or right of the flash bulge (that's intentionally using 'poor quality control'?).
Most T-mount and Adaptall adapters and bellows mounts can be rotationally (mis)adjusted by the nature of their universal design.
The "radially imprecise" bayonet mount with a locking notch for indexing the lens was functionally much more versatile.
H2