Nikon D5 - Manual (Page 218)

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Unified Flash Control
Unified flash control allows the camera and flash unit to share settings.
If a flash unit that supports unified flash control is mounted on the
camera, changes to flash settings made with either the camera or flash
unit are reflected on both devices, as are changes made using optional
Camera Control Pro 2 software.
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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 180K (180,000) 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 page
243). Not available when spot metering is used.
• Standard i-TTL fill-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 fill-flash for digital SLR is
activated automatically when spot metering is selected.