Diopter Adjustment Knob; Eyepiece Shutter Lever - Nikon F4 Instruction Manual

Diopter Adjustment Knob
This knob enables near- or farsighted photographers to adjust
eyepiece diopter within a continuous range of from -3 to
+1.
Pull the knob and rotate it to either direction until focused
image appears sharp, then push back. For diopters beyond
this range, Nikon eyepiece correction lenses for F3HP are
recommended.
Eyepiece Shutter Lever
During self-timer operation, in automatic exposure modes, use
the eyepiece shutter which prevents light from entering the
eyepiece and adversely affecting the automatic exposure meter
reading. Just turn the lever to the left to close the shutter. The
eyepiece shutter is painted red to provide a visual reminder
that it's in use.
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