ITTL and TTL

What is the difference between ITTL and TTL, does anyone know a good link to explain? Thx

I couldn't find any good links, but here is the best explanation I can give:

i-TTL works by firing a pre-flash before the mirror is raised when you take a picture. The meter evaluates this pre-flash, as well as other information (Aperture, ISO, selected AF point, AF distance, Flash exposure compensation) and determines an exposure. The flash then fires at the selected power level after the mirror is raised and the shutter open.

TTL flash however reads the exposure via reflected light off the film to a flash sensor in the mirror box (or the camera meter in the case of the F3). It is more accurate than a pure pre-flash based system, especially in it's late incarnations (which use a pre-flash for initial calculations as well as the flash sensor reading off the film, aperture, selected focus point and and focus distance). But due to the different (and variation between sensors) reflectivity of digital sensors, it doesn't work well for digital, and only 3 DSLR's have used it successfully (Fuji S2 Pro and Pentax *ist D and *istDS).

Thanks Luiz.

What is the difference between ITTL and TTL, does anyone know a good link to explain? Thx

While not particularly aimed at ITTL but rather at ETTL (Canon's version) you can learn alot from http://photonotes.org/articles/eos-flash/. Alternatively, U.S. Patent 6,564,014 has quite a bit of detail but it might be overkill.

Bill

here is a description from Nikon:

(use keyword TTL on Nikon Europe's support site if link should not work)

http://nikoneurope-en.custhelp.com/cgi-bin...amp;p_topview=1