Metering technique is used to determine exposure value of your subject and environment so that your camera can adjust it's settings accordingly for better results. Nikon D90 supports three types of Metering Modes - Matrix Metering, Center-Weighted Metering and Spot Metering which we discussed previously. The built-in flash in the Nikon D90, as well as external flash units attached to the camera, use the same metering modes that are available for ambient light. You can choose one of the Metering Modes available in your camera settings and camera will adjust your flash metering mode automatically unless it is in manual mode. Choice of a Metering Mode determines how the flash reacts to balance the existing (ambient) light with the light from the flash units. Nikon latest cameras and flash units use TTL (Through The Lens) technique to determine the amount of lights in your subject and adjust flash power to balance overall lighting condition for proper exposure. TTL in general stands for Through The Lens and it refers to the process of measuring flash exposures through the lens using a meter inside the camera (as opposed to a sensor on the flash) and controlling the flash duration by the camera. Older camera systems either used manual calculations to determine flash exposure or used a sensor on the flash itself. And i-TTL (intelligent-TTL) refers to the latest TTL flash system from Nikon. As of my understanding TTL metering in new Nikon cameras means i-TTL by default. Basically there are two types of i-TTL method used by Nikon D90.
1. iTTL Balanced Fill-in flash (i-TTL/BL or TTL/BL)
Nikon's i-TTL (intelligent through-the-lens) Balanced Fill-Flash automatically balances the output of the Nikon Speedlight and the scene's ambient light. If balance fill i-TTL is chosen by camera, Nikon Speedlight fires series of pre-flashes to determine exposure value of your subject and environment and send that information back to the camera to fire main flash with correct flash power. Combined with information from the camera's 3D Color Matrix metering system, the information is analyzed to adjust flash output to balance the scene's ambient light. All of this complex processing happens in a fraction of a second, before each exposure, to provide unprecedented levels of flash precision and performance. This works pretty well and Nikon has made it very easy.
This flash mode is used when you chose Matrix Metering or Center-Weighted Metering technique in your camera settings. The Nikon D90 measures the available light and then adjusts the flash output to produce a natural balance between main subject and background. It will reduce harsh shadow and highlights in the subject caused by over flash power.
2. Standard i-TTL Fill-flash
This mode is activated when you use Spot Metering Mode. The flash output adjusted only for the main subject in your frame, and the brightness of the background is not factored in. If standard i-TTL Fill-flash is chosen (by setting Manual or spot metering in A, S, P), the background light is ignored and the camera/flash tend to give you a correct exposure of your subject. You should use this mode when you want to emphasize the main subject at the expense of proper exposure for the background.