Nikon Df - Manual (Page 144)

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i-TTL Flash Control
When a CLS-compatible flash unit (0 118) 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 a
2016-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, 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 0 163). 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.