Nikon D300 - Manual (Page 331)

305
U
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. 302) and [FUNC. button+dials] (pg. 304),
except that the default option for [Preview button press] is
[Preview] and the default setting for [Preview + command dials] is
[None].