Glossary
Preset aperture
A preset aperture uses two rings: one sets a limit, the other opens and closes freely up to that limit, so a photographer can focus wide open and stop down to a chosen value without looking.
Also called preset lens, two-ring aperture
How it works
It predates automatic diaphragms. On an early SLR the viewfinder went dark when you stopped down, so the workflow was: set the limit, open up to focus, then close to the stop by feel and shoot.
Preset lenses have no linkage to the camera body and no spring-loaded pin, which makes them the simplest possible lenses to adapt. Any mechanical adapter works and aperture control behaves exactly as it did originally.
The alternative on later lenses is an auto diaphragm with a pin the camera depresses. On an adapter with nothing to push that pin, the lens can stay stuck wide open unless it has an Auto and Manual switch, or the adapter has an inner flange that holds the pin in.
Why it matters when buying
A preset lens is the safest vintage buy for adapting because there is no pin to defeat. If a lens has an auto diaphragm and no A/M switch, check that your adapter holds the pin before buying.