Nikon D3X - Manual (Page 365)

339
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f5: Assign Preview Button
Choose the role played by the depth-of-field
preview button, either by itself ([Preview
button press]) or when used in combination
with the command dials ([Preview +
command dials]). The options available are
the same as for [FUNC. button press]
(pg. 333) and [FUNC. button+dials] (pg. 336), except that the
default option for [Preview button press] is [Preview] and that
[Preview + command dials] lacks a [Focus point-selection] option
(the default setting for [Preview + command dials] is [None]).