Nikon D90 User Manual page 24

When taking shots at depths deeper than 3 meters, the use of a "flash" is
necessary to regain the natural colours. The Nikon D90 can be combined with
a flash in 3 modes:
Manual flashes: using this mode, the power of the flash can be adjusted.
The camera will direct and synchronise the flash by means of the X-contact.
The result obtained can be observed on the LCD-screen. In case of an over-
or underexposure, the power of the flash or the distance to the subject has
to be adjusted and a new shot has to be made.
Autonomous flashes: some flashes have a light sensor which registers the
light reflected on the subject. When this sensor registers enough light, it
will automatically shut down the flash. Only by means of the diaphragm,
the flash light can be limited. To obtain a correct simulation, the diaphragm
used on the camera has to be adjusted on the flash. In case of an under- or
overexposure, a higher or lower value of the diaphragm has to be adjusted
on the flash.
TTL flashes: using this mode, camera and flash communicate and set the
right values of diaphragm and shutter speed to obtain the correct amount of
light needed for the shot. This mode can only be obtained by using a Nikon
D90 compatible flash or by installing the optional TTL converter.
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