Using 2 strobes-how does optical TTL work?

[bsummers]

I am thinking of making the switch from a one strobe system to a 2 strobe system. I previously have only shot using the strobe set in manual because that was all my strobe was capable of. ( ys 27DX). But now I am intrigue by the possibility of using optical TTL with ys110a and thinking of getting either one or two of them.

I read this on uwphotographyguide.com :

"When TTL is used with strobes, a pre-flash is fired first to measure exposure. The result of the pre-flash is analyzed, and the proper duration of the full strobe output is calculated. "

I don't understand how that could work with 2 strobes ( eg two ys110a) set to optical TTL? Won't you end up with double the amount of light needed and hence an over exposed photo?

Am I misunderstanding this? Is the preflash performed by only the internal camera strobe and by both the internal and external strobes?

Please help!

 

[jdandvalerie]

I shoot with dual Ike 161s using TLL (with an Ike housing). As I understand it, both strobes fire the preflash when you have the dual sync cord providing your system supports TLL thereby giving the proper metering.

 

[bvanant]

In optical TTL there are two ways to do this depending on your housing. If you have a housing that has two optical bulkheads (like Nauticam) then you can use two separate optical fiber cables. If you have only one bulkhead then you will need a single optical fiber that has a Y to go to two strobes. Set your camera up in TTL mode (not manual) and it when the camera sends out a pre-flash, the preflash signal goes to the strobe. The strobe then puts out its preflash and the camera reads the amount of light that comes back to the sensor. Then the camera sends out its flash at the appropriate power to light the scene. The camera flash is then seen by the strobes which send out their flash at the appropriate power. The key is that the strobe (one or two) output is proportional to the output of the camera. Mostly this works out fine; sometimes with wide angle shots upward into the sun the camera can get fooled but for macro this works great.
Bill