I'm relatively new to strobe shooting, so I mostly stick to TTL. I'm guessing the ultimate answer to this question is: use Manual instead of TTL. Nonetheless, there is a theoretical issue I've been grappling with since I started researching strobe shooting and reading as much as I could.
As far as I understand things, the idea of TTL in an optically triggered external stobe is something like this: the camera figures out how much flash to use and fires the on-camera flash. That flash is completely blocked by the housing, only traveling on the fiber to the strobe, which is then able to see how much flash the camera felt was needed and set the output of the strobe to match.
However, there is a lot of advice for improving on-camera flash recycle times which suggests decreasing the output of the on-camera flash. On my camera I believe this would be called Flash Exposure Compensation (fec). This is entirely logical. Since the on-camera flash is not illuminating anything, why not set it to as low as possible while still triggering the strobe. This should save battery life and improve recycle times.
What I don't get is this - how could dialing down the fec not affect the TTL communication between the on-camera flash and the strobe? It seems like you're manually overriding a camera-set value. This would in turn yield inaccurate TTL results.
Perhaps I'm missing something very basic? That's very, very possible...
550d/Sea&Sea RDX 550d/1x ys-110a