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Can ELI5 how S-TTL works?

Sony Rx100 S-TTL TTL Inon Z-240 Rx100M4 ELI5

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#1 Photonphillic


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Posted 18 October 2017 - 11:55 PM

I'm very new to using an underwater strobe and don't really understand how a TTL system works using a fiber optic connection. I've been watching youtube videos explaining TTL in normal cameras connected via a hotshoe with an electronic connection and kind of understand that the camera fires a pre-flash to determine how much power it needs for the main flash. But how does this work with a simple optical trigger? If the camera uses its own pre-flash to trigger a pre-flash in the strobe and then provides corrects the main flash power, how is this information conveyed via the fiber optic from the camera to the strobe? To clarify, does the strobe vary its power according to the amount of light carried via the fiber optic cable?


I have a Sony RX100M4 connected to an INON Z-240 (unfortunately there's no manual) via optic cable.


Does anyone know if the RX100 uses a preflash when I use the fill flash? Should I put the Advanced Cancel Circuit Switch up or down on the Inon Z-240?


Any help would be very appreciated. 



#2 Vondo


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Posted 19 October 2017 - 05:41 AM

This should help: https://reefphoto.com/kb.php?id=7Even if you had the manual, you'd probably find it almost incomprehensible. It's horrible.


TTL works by mimicking the TIME the strobe is firing. 


I've always just left the cancel circuit turned off because I usually shoot manual and definitely don't want to worry about going back and forth when I do shoot TTL.

#3 Kraken de Mabini

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Posted 19 October 2017 - 09:13 AM

  You ask an interesting question with a complicated answer. 

First, as I understand it, different makes of digital cameras use different TTL light codes, and these codes are patented and proprietary.  Nikon, Olympus, Canon, etc all use different TTL light codes.


Second, TTL consists of a series of light pulses of varying length, which are reflected from the subject back to the camera.  The camera interprets the reflected pulses and adjusts its final flash duration accordingly. 

When the diver's camera TTL light pulses go through a fiberoptic cable to the strobe, the strobe re-emits them, so that the camera sees the reflected stobe's pulses and interprets them as if they were the camera's own TTL light pulses.  It then flashes its definitive, photo flash, which goes through the fiberoptic cable to the strobe.

The strobe should then emit a flash of a duration appropriate to take a well exposed photo. 


All this is very nice, but getting it to happen is not easy.  That is why most, if not all, the really good, and also the not so good, underwater (UW) photographers I know use Manual settings.  Why is this?  Because the UW strobe market is small and is ignored by the camera makers.  The proprietary nature of the many TTL pulse codes makes it most difficult for the makers of UW strobes to include the appropriate circuitry in their strobes. What to do?


The current solution is to use an intermediate adapter in line between the camera and the strobe, to translate the camera's light pulses into a sequence the strobe can use to emit the correct photo flash.


ReefPhoto has done us a valuable service with its summary of  available:  ReefPhoto http://reefphoto.com/kb.php?id=105. 


Backscatter also lists available adapters: http://www.backscatt.../Flash-Triggers


Suggested Settings: For TTL on your Inon Z240 strobes set the power knob to TTL, the EV knob start at 12 o'clock, adjust as needed, and the Cancel Circuit in the Out position.


For Manual, the Power is set to M, the EV is adjusted accordingly, turning it clockwise for more light, and the Cancel is pushed In and locked to prevent preflashes.


With the Sea&Sea YS-series, do not use TTL, as the strobe will overheat and may go Bang! as narrated here in Wetpixel.  Use only Manual.


Good luck and Great Photos!

Also tagged with one or more of these keywords: Sony, Rx100, S-TTL, TTL, Inon, Z-240, Rx100M4, ELI5