The introduction to Through The Lens (TTL) Light metering and its consequences on the lens mount
Now that the instant return mirror and the preselection mechanism of the diaphragm had made SLRs usable for action photography, the manufacturers managed to address the next challenge: the determination of the exposure.
Nikon was very proud a few months ago when the 50th anniversary of the F mount was celebrated. Half a century! Pentax had to abandon its original mount and transition to a new bayonet in the early seventies, Minolta and Canon in the mid eighties.
But there is more to lens and body compatibility than the design of the bayonet.
Even if the current Nikon bodies and lenses still use the same bayonet design as the Nikon F of 1959, it’s practically impossible to pair an unmodified lens from 1959 to a recent body, and vice versa: the lens and the body of a modern SLR have to exchange information and commands, and non-upgraded lenses from 1959 simply don’t share enough information to be usable.
The transmission of information from the lens to the body – focal length, maximum and minimum aperture, pre-selected aperture, focusing distance, and of commands from the body to the lens – setting the focusing distance, setting the aperture value, closing the diaphragm, can be performed from many different ways – some of them passive (a hole in the metal), some of them mechanical (rods, cogs and springs), the most recent working exclusively through electrical contacts.
Cameras of the mid fifties were far less complex than the ones we now use. No internal meter, no auto exposure, no auto-focus.
But users of SLR cameras were facing an important issue: because the viewfinders of their cameras were dim and the focusing screen grainy, the only practical way to set the focus was to open the aperture to its maximum. Let’s say F:1.4. But if on a sunny day they needed to shoot at 1/125 sec at F:11, they had to set the aperture ring to F11 AFTER they were finished with the focus and – of course – BEFORE they took the picture. Not very fast, not very convenient.
At the end of the fifties, most Japanese camera manufacturers adopted automatic diaphragms with aperture pre-selection: the lens remained at full aperture – let’s say F:1.4 -independently from the aperture value selected by the user on the aperture ring, making focusing easy. Only when the user pressed the shutter release to take the picture would a lever or a rod mechanically close the diaphragm to the value pre-selected by the user.