Nikon i-TTL, short for intelligent through-the-lens, takes advantage of modern technology that will automatically determine the amount of light projected by your flash to get a correct exposure on your digital camera. It works by emitting a pre-flash as you press your camera's shutter. The pre-flash measures the distance of your subject through the lens, which allows your camera to compute the correct amount of flash light to match your shutter speed and aperture.
Prior to I-TTL
Until Nikon unveiled through-the-lens technology for film cameras in the 1980s, you had to learn manually estimate distance to subject and adjust your camera's settings based on a numerical chart. It took more time to correctly determine exposures using a flash, which sometimes could lead to missing critical moments.
How it Works
I-TTL simplifies using a flash in complex or rapidly changing lighting situations. It's especially helpful when using fill flash to lift shadows in harsh sunlight or when your subject is back-lit. Nikon i-TTL flashes offer a back-light mode in which a pre-flash bounces off every subject in the frame. Your camera processes the returned information to compute the correct exposure to light your subjects evenly.
Drawbacks
I-TTL doesn't always work perfectly. Dark-toned or high-contrast subjects can cause your camera to get an incorrect exposure from your flash. The solution is to bracket your flash exposures, which means to take several different exposures of the same photo in rapid succession. Nikon i-TTL flashes feature the ability to adjust power output while in TTL mode, allowing you to bracket quickly for the perfect exposure.
Flashes With I-TTL
Nikon offers a range of i-TTL flashes and compatible digital cameras that vary in power, price and features. On the low end, the SB-400 flash costs about $120, as of April 2011. Its head can swivel only horizontally. The SB-600, at $240, offers full tilt and swivel. The SB-700, $320, and SB-900, $470, both offer more power, and they can serve as commander units to set off other flashes in Nikon's Creative Lighting System.
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