Nikon EL2 - Manual (Page 65)

until it clicks and locks into place. These steps provide
for full mounting of the lens, while simultaneously
indexing the lens’ maximum aperture setting to the
camera’s metering circuit.
To mount a lens not fitted with a meter coupling ridge,
first lock the meter coupling lever in the up position
(again, see “Coupling Lever Lock/Release Operation”
for details). Then mount the lens and lock it into posi­
tion as explained previously. For operation with lenses
not fitted with a meter coupling ridge, stop-down mea­
surement (as described on page 32) is required.