To Confirm Sharpness Before Shooting - Nikon N90 AF Instruction Manual

TO CONFIRM SHARPNESS BEFORE SHOOTING-DEPTH-OF-FIELD PREVIEW BUTTON
Note that when you're using a lens with an automatic
diaphragm, which is what most Nikkor lenses have, the
viewfinder image you see is one with the lens fully open (i.e.,
lens at maximum aperture). This enables you to view the depth
of field before you take the picture, with the N90's Aperture-
Priority auto or Manual exposure mode.
Depress the depth-of-field preview button to stop the lens
down to the aperture set. The image viewed becomes
progressively darker as the aperture gets smaller. Portions of
the picture that appear in focus when the button is pushed
down are within the depth of field.
While the depth-of-field preview button is depressed, shutter
speed indication disappears and
F- -
appears in the LCD
aperture readouts ; also, the shutter is locked.
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