How does TTL work with just a fibre-optic cable?

Heya. I have inherited a venerable YS01 strobe which has just two power options - ON or TTL. It is being triggered by a simple fibre-optic cable attached by Velcro sticky pads to the Ikelite Compact housing and in front of the strobe trigger sensor. I'm getting decent enough results with ON (i.e. manual, full power) and playing with distance, strobe positioning, ISO, E/V, f/number and shutter speed, but don't notice any difference when I put the strobe in TTL.

My understanding from the strobe manual is that in TTL the strobe assesses the scene and adjusts the flash light output accordingly, and if it cannot decide, it just punts out of full power. Question is, if it does decide to vary the power from full, what aperture and shutter speed settings is it assuming the camera is using? There is a guide number chart in the manual - is that what I need to refer to in order to set the camera up? Clearly the strobe is not getting this information down the fibre-optic cable.....all that is doing is telling it to fire.

It's a question I get asked often by diving guests where I work, so it would be good to know what TTL does in this set-up.

I assume also that using a bulkhead connector between the camera's hotshoe and the strobe allows exposure information to be shared with the strobe? (my camera does not have a hotshoe, but I'd like to know anyway for future purchases). Thanks

can you confirm the manufacturer and model of the strobe? your control description does not match a sea &sea ys-01...

if a strobe is being used as an optical slave, it will mimic the operation of the onboard camera flash. the camera will see light from both flashes and perform ttl. when the onboard flash stops the external strobe stops. in this case the strobe has no ttl smarts. it just starts and stops in concert with the onboard flash. some strobes are better at this than others...

if a wired sync cable is being used then the camera controls the strobe with several wires (X, Q, Gnd) no ttl logic in the strobe. it just starts and stops when told by the camera. the strobe has no knowledge of any camera settings or exposure.

some strobes also have a "simulated ttl" featre where you set an "exposure level" (as opposed to a power level) and the strobe uses a sensor to detect reflected light and decides when to stop. this is not ttl as the strobe has no idea how much light has fallen on the camera lens. it can only detect what has been reflected back to the strobe. the strobe also has no knowledge of your camera settings.

Thanks for replying. My mistake, the strobe is a ys-30 duo II TTL. The sync cable is a simple fibre optic one to pass the flash from the camera to the sensor on the front, in which case the strobe fires. As I said, I've been using the strobe as a slave, and setting the flash output of the camera to minimum to preserve battery life, just using it as a trigger for a full power strobe discharge. Are you are saying that if I put the strobe into TTL I would also need to put the cameras flash into automatic so the duration of that flash could be mimicked by the strobe?

So TTL in this context does not mean that the strobe emits a preflash of its own to determine power, it relies on the camera to do the measuring of the incoming light from both camera flash and strobe, and stops the camera flash and therefore the strobe when correct exposure has been achieved?

Your last paragraph got it right. The camera detects when the correct exposure has been achieved and then quenches the strobe. To use TTL both the strobe and camera have to be set to TTL and, to my knowledge, you can't reduce camera strobe power in that case. Actually I don't think the strobe can change its intensity, only its duration.

Bart

Thanks again, all clear now. Next time out I'll try putting camera on auto-flash (i.e. TTL) and the strobe on TTL and see what happens.

With some strobes that has +/-EV settings in TTL, you can theoretically decrease the camera's internal flash power and still get a proper exposure. If you set your camera's TTL flash power to -3EV and your strobe to +3EV, the camera will send a pulse of flash 1/8 the duration of a proper exposure when taking the picture, but the external strobe will make that pulse 8X longer, thus properly exposing the subject again.

Why use TTL? It's vastly not accurate for digital photography. It just over expose all the time.. Then they tell u to adjust the strobe to the right light output. Why not just put to manual and just adjust from there...

Why use TTL? It's vastly not accurate for digital photography. It just over expose all the time.. Then they tell u to adjust the strobe to the right light output. Why not just put to manual and just adjust from there...

Just to put another side to this, I've been using TTL for about 5 years with a Nikon D300 and then a D800. With the D300 I used Inon Z240 strobes and a Heinrich Weikamp TTL convertor; with the D800 I use the same Inon strobes and fibre-optic triggering off the camera's built-in flash.

You can indeed use flash manually and by-and-large it works pretty well. It's not hard to adjust, you can usually figure out before firing what the exposure settings should be; and it can be less complicating in terms of set-up and calibration - although I find this less the case now with fibre-optics and camera built-in flash.

You can hear the "but" coming...... I did/do find though for macro, TTL is terrific. It's rare that the exposure is incorrect and, for me, I know that if the flash firing is going to scare off a skittish critter, chances are that a one-off exposure (at least!) will be right with TTL. Not to say of course that it'll be perfectly in focus etc etc.

So, given the choice, I'll take TTL and when I moved from the D300 to a D800 setup I made sure that I could continue to use a facility I had really come to appreciate with the D300/Heinrich Weikamp.

I have had similar experiences as Tim with TTL, that it works very well with correct exposures and I shoot TTL almost exclusively.

I started with a D200 with a Sea & Sea TTL, then went to a D2Xs and now with a D800E. I continued to shoot with the Sea & Sea TTL until it died last year on the Komodo Dancer. My back up Sea & Sea TTL that I have been carrying since 2006 died on its first dive in the rinse tank. I then went with fiber optics with the pop up flash and continued to have good exposures. The electronic TTL allows one to take more shots in a rapid sequence, where after a few shots with the pop up one needs to wait for the flash to recycle. Using either electronic TTL or pop up flash TTL my exposures have been consistently correct.

Elmer