Nikon EL2 - Manual (Page 60)

MIRROR LOCKUP
The reflex mirror must be locked in the up position
when using either the Fisheye-Nikkor 6mm f/5.6 or the
OP Fisheye-Nikkor 10mm f/5.6 lenses, since their rear
elements protrude into the camera body and interfere
with mirror movement. Locking up the mirror is also
necessary when changing the battery mounted in the
chamber at the bottom of the mirror box. To lock up
the mirror, turn the mirror lockup lever upward as far
as it will go (approx. 45°). The mirror will remain in the
up position until the lever is returned to the normal po­
sition.
Note; The mirror lockup lever function can be duplicated by
using the self-timer to trigger the camera, as the timer action
causes the mirror to swing up at the start of the countdown
process. This is convenient particularly when minimization of
mirror shock is desired, without the need to actually lock the
mirror up, as in photomicrographic applications.