Mirror Lockup Lever; To Lock Reflex Mirror In Up Position - Nikon F5 Instruction Manual

MIRROR LOCKUP LEVER—

When using super-telephoto lenses or performing photomicrog-
raphy, it is necessary to reduce camera vibration to the absolute
minimum. Lock the reflex viewing mirror in the "up" position by
rotating the mirror lockup lever counterclockwise until it stops.
When the reflex mirror is locked in the up position, make
sure not to leave the camera in direct sunlight. The sunlight
may enter through the lens and may damage the shutter
curtain.

To lock reflex mirror in up position

With the mirror lock up, you cannot operate the camera in any
Auto Exposure or autofocus mode, even though the viewfinder
LCD may indicate otherwise. Any indication of light in the LCD is
a result of spurious light entering through the viewfinder
eyepiece.
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