TTl stands for 'through the lens' and it is the way the camera meters for exposure. When flash is involved, the TTL mode of a flash (whether its on or off camera) means that during the exposure (ie when you press the shutter), the following happens;~ the camera 'tells' the flash to fire a pre-flash, ~ the camera measures the exposure and then decides what flash level the output needs to be to provide a 'correct' exposure and,~ it tells the flash to fire at this output levelAll this happens in a fraction of a second during the pre-flash time. iTTL is Nikons proprietary name for its TTL system & it stands for 'intelligent TTL'.The SB600 will work as a iTTL flash for Nikon cameras only, it will not work as a TTL flash with any other make of camera (although it can be used in manual mode where you set the power output manually).With higher end Nikon cameras (D70, D80, D90, D200, D300, D700) the SB600 can also be triggered & controlled wirelessly off camera via the in-camera menu and pop-up flash on the body.

TTL is the generic term for Through The Lens flash exposure metering. Basically it uses a preflash to automatically determine the power of the flash.i-TTL is Nikon's current proprietary system, called intelligent TTL, basically does the same thing as mentioned with Nikon's own clever firmware.TTL is also the original name for Nikon's metering system in the film days, this evolved into D-TTL and later i-TTL. In TTL mode the flash may not expose properly with DSLRs. Most flash systems had to change when we transitioned from film to digital, and as a result there is this enforced incompatibility.http://photo.net/equipment/nikon/guide-t…