Whilst digital cameras quickly took over film cameras for land photography, spread among underwater photographers took longer. Main reason was incompatibility of underwater strobes with digital TTL Auto making hard to benefit fully automatic strobe shooting.
INON is among the first to support digital TTL auto system with S-TTL Auto mode equipped D-2000 strobe and Z-240 strobe.
The proper name of the S-TTL is Optical Synch TTL. The S-TTL enables the strobe to perform in TTL auto exposure as same as genuine TTL strobe from camera manufacture, based on camera’s built-in flash light to use as a signal to transmit it to the strobe. The S-TTL uses the built-in flash not as a light source but as like a controller to trigger the S-TTL strobe.
The built-in flash of a digital camera makes weak flash (pre-flash) before main-flash to calculate exposure. This pre-flash is transmitted through fiber optics to an S-TTL strobe to control the strobe to make pre-flash at a subject. The reflecting light from the subject goes through the camera’s master lens to an image sensor then a processor determines main flash light amount for correct exposure. Finally the built-in flash makes main-flash which is transmitted to the S-TTL strobe to cause main-flash of the S-TTL strobe.
Digital camera’s built-in flash lights are transmitted via an optical fiber to the
S-TTL strobe to make pre-flash and main-flash instead of the built-in flash.