TTL (through the lens) flash systems measure the light output via an in-camera sensor to get correct flash exposure. Companies have different versions of TTL exposure. The latest Nikon version of TTL is i-TTL (intelligent through the lens).
Camera flashes
How It Works
With i-TTL, Nikon fires a monitor preflash that helps the sensor measure the available light for exposure. With i-TTL, the flashes can be picked up by either the five-element TTL flash sensor or an RBG (red, green, blue) sensor to get the right reading.
Balanced Flash Exposure
With i-TTL, the monitor preflash reflects off of every surface in the frame of the picture the photographer has composed. The information the sensor reads is combined with output from the camera's Matrix Metering System to give a balanced flash exposure for the whole frame, both foreground and background.
Flash Units
Nikon currently makes four flash units with i-TTL flash built in: the SB-900, the SB-600, the SB-400 and the SB-R200. The R1 and R1C1 ring flash systems for close-up photography also use i-TTL via their included pair of SB-R200s.
Legacy
Nikon introduced its i-TTL system to its film SLRs in the 1980s. With the advent of digital SLRs, Nikon created the D-TTL system, which was supposed to work better with digital by flipping up the mirror but not firing the shutter. D-TTL was used from 1999 to 2003. It was phased out for a new, improved i-TTL.
i-TTL with CLS
In 2003, Nikon announced their newest DSLRs would merge i-TTL with the new Creative Lighting System (CLS). The new i-TTL measures the pre-flash inside the viewfinder before the mirror flips up. CLS allows the photographer to create three groups of remote wireless flashes with different flash exposure values that can all be triggered by one master flash.
Tags: flash exposure, With i-TTL, flash systems, i-TTL with, monitor preflash