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.
LINKS
» Preset Aperture Lenses – How They Work And Why You Need At Least One
By Retro Photo House