mesure ouverture utile

OBS: [...] si, face à une source lumineuse, on applique à l'arrière de l'objectif un verre dépoli, il se forme sur ce dernier un cercle lumineux dont le diamètre est l'ouverture utile dite aussi ouverture mécanique.

CONT: Fortunately, you can still meter using the stopped-down metering approach on most cameras. By pressing a depth-of-field button, the camera is placed into stopped-down metering mode too. The lens is stopped down to the taking aperture automatically on some adapters. On T-mount and other purely mechanical mounts, you may have to turn the aperture ring to the desired taking aperture. In either case, the camera is now metering at the actual stopped down aperture. You can set the shutter speed and aperture as desired, based on your meter reading.