The FL Series Lenses
On the back (to the left) of this lens you can see the “Breach Lock Ring”. On the front (to the right) you can see the silver aperture setting ring.
The single pin on the back of an FL lens is shown. Note how uncluttered the back of an FL lens looks. The locking lugs on the Breach Lock Ring are clearly visible.
The distinguishing features of this lens, aside from the “FL” letters inscribed in the front lens ring, are the two silver rings, the Breach Lock Ring and the Aperture Selection Ring, that are visible in the picture to the left and the single Automatic Aperture Pin protruding from the back of the lens. It is easily recognized.
The desired f/stop is selected on the Aperture Selection Ring but the lens remains wide open. When the shutter is pressed the camera body has a lever that engages the Automatic Aperture Pin and the lens shuts down to the previously selected f/stop. Then the shutter curtain is released, the exposure is made, and the lens reopens to full aperture. It happens in the blink of an eye.
This predates automatic exposure settings and so the only information communicated to the lens is that the shutter is about to fire and the lens must stop down. Thus a single pin suffices.
The neat clean appearance of the rear surface of the lens is a clear giveaway that this is an FL lens. So now you too can recognize what you are looking at when you hold an FL lens at a camera show!
I have more than you see here because it takes time and work to get them all listed.