Nikon D300s - Manual (Page 196)

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The Built-in Flash
The built-in flash has a Guide Number (GN) 17/56 (m/ft., ISO 200,
20 °C/68 °F). It supports i-TTL balanced fill-flash for digital SLR,
which uses monitor pre-flashes to adjust flash output for balanced
lighting not only when natural lighting is inadequate but when
filling in shadows and backlit subjects or adding a catch light to
the subject’s eyes. The following types of i-TTL flash control are
supported:
i-TTL balanced fill-flash for digital SLR : Flash emits series of nearly invisible
preflashes (monitor preflashes) immediately before main flash.
Preflashes reflected from objects in all areas of frame are picked up by
1,005-pixel RGB sensor and are analyzed in combination with range
information from matrix metering system to adjust flash output for
natural balance between main subject and ambient background
lighting. If type G or D lens is used, distance information is included
when calculating flash output. Precision of calculation can be increased
for non-CPU lenses by providing lens data (focal length and maximum
aperture; see
0 195). Not available when spot metering is used.
Standard i-TTL flash for digital SLR : Flash output adjusted to bring lighting in
frame to standard level; brightness of background is not taken into
account. Recommended for shots in which main subject is emphasized
at expense of background details, or when exposure compensation is
used. Standard i-TTL flash for digital SLR is activated automatically when
spot metering is selected.
D ISO Sensitivity
i-TTL flash control can be used at ISO sensitivities between 200 and
3200. At values over 3200 or under 200, the desired results may not be
achieved at some ranges or aperture settings.