Warren_L:
iTTL is Nikon's TTL implementation for the D70S. TTL measures the amount of light for a shot and automatically shuts off the strobes or flash for the proper exposure. It measures the light Through The Lens (TTL).
To add to this, most DSLR's use TTL metering, in fact I can not think of one that does not. So what you have when you add flash is a camera that uses a built in reflective meter that measures both ambiant AND flash light at the sensor plane. When you add flash, then there is a seperate part of the built in TTL system that measures, and controls the flash via the TTL metering system. In the case of Nikon that is called iTTL.
Your D70's built in flash uses iTTL to control the output.
Most systems use a series of preflashes to try and determine what level the flash needs to be set at. There are a few modes, but broken down most camera's either attempt to balance the flash with ambiant light, or they rely on the flash as a primary light source. Balancing ambiant and flash is generally defined as *fill* flash.
A VERY useful tool in setting flash via iTTL is the flash exposure compensation. Reflective meters are easily fooled, in addition strobes produce harsh flat light. One way to gain a more natural look, and to prevent overexposed highlights when using flash is to back down the flash exposure. So I often shoot with a flash exposure compensation of -2 stops. That allows the flash to fill in shadow area's without really impacting the overall natural light.
This technique can be used with a manual flash as well, but one has to often mess around a bit to get the flash power at a setting that is approximately 2 stops or so less then the ambiant light. iTTL is nice as it generally works well without having to measure ambiant light, and the flash output seperately, or with a ambiant light meter. If shooting in a dynamic environment when light conditions are changing (for example a sunny day with lots of cloud cover so the sun goes from bright to blocked often) iTTL is really nice. If shooting in an environment where the light is constant, manual shooting works very well as one can set it, and basically forget it as if light conditions remain constant, so will the manual flash output, and exposure in camera.
I could likely write a book on reflective and ambiant meters, and their shortcommings. One thing to keep in mind is that reflective meters are easily fooled by very bright, or very dark subjects. This is where the manual settings can work better than allowing the camera to just do it as if one get's the manual flash and exposure set correctly for a nuteral scene in a given light then the reflective meter is not adjusting for highly reflective or dark situations in error.
