Nikon D300 User Manual (en)

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 
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.
i-TTL balanced fill-flash for digital SLR
: Speedlight 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-segment 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 pg. 196). 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.