Nikon D4S - Manual (Page 224)

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i-TTL Flash Control
When a CLS-compatible flash unit is set to TTL, the camera
automatically selects one of the following types of flash control:
i-TTL balanced fill-flash for digital SLR : Flash unit 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
RGB sensor with approximately 91K (91,000) pixels 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, E, 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
0 235). 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.