On the FE there is a meter coupling tab located at one-o’clock on the mount. After 1977 all have this tab. It replace the meter coupling prong on the lenses. These prongs coupled with the modular photomic metering system in 1962.
But with the meter coupling lever allowed automatic indexing. No more teaching the metering the lens’ maximum aperture is. Faster lens changing. Aperture priority is now an option.
Lenses made after 1977 utilizes AI and so the lenses were designated AI lenses. And lenses made before AI lenses were logically pre-AI or non-AI lenses.
A new problem arose with this. Non-AI lenses did not have the ridge into which the meter coupling tab fitted into. So trying to mount a non-AI lens onto a mount that has the tab (like the FE) would damage the body, lens, or both.
The solution was to make the tab non-fixed. Hence the folding meter coupling tab.
Only bodies made in 1977, FE (1978), F3, F4, and Df (digital full frame) had this folding meter coupling tab.
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