Nikon d3s User Manual (en)

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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 
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 
0 214). 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.