Preset aperture lenses

The preset aperture mechanism — a click-stopped ring to set the stop-down aperture accompanied by a stopless ring that smoothly moves between wide open and the aperture preset on the other ring — has several advantages for digital cameras:
– Digital cameras generally can’t automatically stop-down a manual lens when you press the shutter button, so they behave exactly like the film bodies that were expected to be behind preset lenses.

– I’ll often preset the aperture at the smallest aperture that will not degrade the image quality by diffraction or sharply-imaging dust, then just use the stopless ring to vary within the acceptable range.

– The stopless ring allows changing the aperture during videos without steps or click noises.