Pentax 645 - Depth-Of-Field Preview

DEPTH-OF-FIELD PREVIEW
Should the lens possess an automatic diaphragm,
the viewfinder can be kept bright with the
aperture fully opened regardless of the preset
f/stop. When the preview lever is depressed in the
direction of the arrow, aperture will stop down to
the preset value, and the depth-of-field can be
confirmed through the viewfinder. The aperture will
return to maximum when the preview lever is
released. The depth-of-field cannot be confirmed
while the lens is set to '"A".
Note: Exposure value measured during preview
may register incorrectly. While depth of field is
being previewed, the shutter cannot be released.
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