Digital Camera Fully Compatible, Newly Developed S-TTL System
"S-TTL" enables TTL auto shooting by an external strobe for a digital SLR camera as well as for a point & shoot digital camera.
INON S-TTL auto strobe supports any manufactures' model providing highly accurate exposure control.
Film camera era without strobe selection problem
TTL stands for "Through The Lens" and TTL auto strobe system controls flash amount to provide correct exposure based on calculation by camera's internal sensor metering reflecting strobe light from a subject through the lens. This TTL system meters actual light amount reflecting from a subject providing accurate exposure.
When we start with the history of underwater TTL auto strobe, underwater camera?"NIKONOS V" released in 1984 was the first to provide automatic TTL flash control for underwater strobe SB-102, SB-103 succeeded by NIKONOS V compatible underwater strobes form other manufactures. The 5 pin electrical sync connector for NIKONOS V is most popular and widely adopted to connect an underwater strobe and underwater film camera (underwater camera/housing).
A film SLR camera has flexibility to select an underwater strobe. As far as housing has NIKONOS type electrical sync connector and properly wired, automatic TTL flash control is usable with any TTL auto strobe like Nikon SB-105, INON Z-220, Z-22 connected via electrical sync cable.
NIKONOS type 5 pin electrical sync connector and NIKONOS V with INON Z-22 strobe
Film camera compatible strobe is not usable for a digital camera!?
Underwater TTL strobe circumstances have been drastically altered with the spread of digital camera among divers. Some underwater housing for digital SLR camera has NIKONOS type electrical 5 pin sync connector as same as film camera housing. Since the housing has same NIKONOS type sync connector, existing film camera compatible underwater strobe has been considered compatible with a digital SLR camera via 5 pin sync cable but happened to experience error message on the digital camera or blackout on an image even the strobe worked. Why this happened?
This is because automatic TTL strobe system difference between a film camera and a digital camera.
The film TTL auto system makes single flash while digital TTL auto system employs pre-flash type making two to three flashes. Film TTL auto strobe system starts firing at the same time the shutter opens and reflecting strobe light from a subject is recived at a film. The reflecting light on the film is metered by the sensor to determine when to cease firing for correct exposure. This process takes only about 1/1000 second.
In contrast, digital TTL auto strobe system can not calculate adequate exposure based on reflecting light from a subject since image sensor (CCD/CMOS) reflectivity is comparably low comparing to a film. So digital TTL strobe system gives preliminary flash for very short period of time (pre-flash) just before the shutter opens metering reflecting light from a subject by the sensor to calculate necessary amount of light for correct exposure and starts emitting main flash at the same time the shutter opens. Canon E-TTL and Nikon i-TTL employs this system.
When we connect conventional film camera compatible TTL strobe to pre-flash type digital SLR via sync cable, the strobe makes full dump by pre-flash signal then the shutter opens before the strobe has been fully charged resulting in quite under exposed image only with ambient light. Even two time flash compatible strobe like INON Z-220 strobe, does not fully support a digital TTL and force to use Manual flash mode.
Film SLR:A strobe starts firing at the same time the shutter opens and quenches firing when correct exposure has been obtained.
A film compatible TTL strobe connected to a pre-flash type digital SLR via electrical cable flashes does not support TTL auto exposure resulting in synchronization only with first pre-flash or totally uncontrollable.
The birth of digital fully compatible S-TTL
S-TTL auto compatible INON D-2000 strobe and Z-240 strobe works in TTL auto mode by simply setting their main dial to S-TTL position.
A film camera seems to depend more on photographer's skill, experience and feel since the camera does not allow checking images on site. A digital camera allows us to check images right after shooting and enable to try to shoot again as much as we like. And high capacity memory card further allows to shoot hundreds of images. The digital camera makes entry level of underwater photography getting down and nowadays more people buy a digital SLR camera and underwater housing even they have just started underwater photography. However underwater photography gets more difficult because underwater strobe does not work in TTL auto.
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 S-TTL is "Optical Synch TTL". S-TTL enables 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 be transmitted to a strobe. S-TTL uses digital camera's built-in flash light not as a light source but as like a controller to trigger 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 to S-TTL strobe to control the strobe to make pre-flash to a subject. The reflecting light from the subject goes through the camera's master lens to an image sensor then a processor calculates 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 theS-TTL strobe to make pre-flash and main-flash instead of the built-in flash.