Do TTL flashes always "count" as a single pre-flash?

Question

My Sony speedlight will only trigger off-camera with a TTL signal.

I'm looking to expand my controlled-lighting arsenal with other speedlights and monolights that all have the ability to optically slave and ignore a "preflash."

For optical slaves does the camera's TTL flash signal always count as a single preflash, or is it possible that it will be perceived by some optical slaves as more than one preflash?

In the latter case I'll just have to give up on the idea of using the Sony speedlight off-camera in conjunction with other optically-triggered lights.

Answer 1

Basically, you're talking about two different types of optical slaving/triggering, and they tend to be incompatible with each other.

The system that your Sony flash/camera uses to trigger the flash off-camera is a proprietary wireless TTL system. This involves multiple pre-flashes, because the camera is actually communicating most of the hotshoe/foot protocol to the flash over a series of light flashes: communicating the TTL preflash/metering information, setting things like the flash power, setting the sync mode, zoom, and the sync signal itself. Think of it like morse code blipping to communicate a packet of information.

By contrast "dumb" optical slave systems are a simple see-flash-fire-flash scheme. A sensor trips the flash. Sometimes, this type of slave can also be set to ignore a single preflash (i.e., fires on the second burst it sees). This is because if you use non-wireless TTL, there is only a single preflash that the camera meters in order to set the flash's power. So some "dumb" slaves can be used with an on-camera or pop-up flash that's set in TTL. But this supposes that the wireless-TTL communication scheme is NOT being used.

There are a number of flashes that have this "dumb" slave built in—Nikon, Metz, Nissin, Yongnuo, Lumopro—a lot of 3rd party and particularly manual-only flashes have this feature built in. You can also add them on—typically via the PC port or the flash's hotshoe [and depending on which Sony hotshoe your flash has, this may also require a hotshoe adapter]. But the main problem with this kind of triggering is that it's "manual-only"—that is, the only signal that's communicated to the remote flash is the one to fire at a certain time. There is no control over the power. No TTL capability. Possibly no 2nd curtain capability. No way to set the zoom. Everything must be set directly on the remote flashes themselves.

In addition, optical slaving has weaknesses: namely, range and line of sight. The sensor panel of the flash has to "see" the mastering burst that's telling it to fire—the flash cannot be blocked from that flash, and must be facing towards it—or towards a bounce surface that can relay the master signal. So, placing the flash behind the camera if your pop-up flash is the master can be problematic. Shooting outdoors without bounce surfaces can be problematic. Modifiers can be problematic, and you're more limited on where you can place the flash. Outdoors in bright sunlight the master signal may also be made fainter by the strength of the sunlight, so the range gets smaller.

This is why radio triggers are preferred by most people who do off-camera flash. The line of sight requirements go away, and the range and reliability are much better than with optical triggering systems, and these days, radio triggers cost just as much as optical slaves and hotshoe adapter combinations.

Answer 2

Out of curiosity I cranked my GoPro 3+ up to 240fps and recorded the TTL sequence between a Sony Alpha and the Sony HVL-F43AM (shown here at 1/16 speed).

The initial flash message from the camera, consisting of some series of flashes too fast to discern at this framerate, runs for 1/40 second. The speedlight metering flash fires 1/24 second later. 1/8 second after that the camera flashes another 1/40 second burst of data, and the exposure flash fires 1/40 second after that.

I also tried using the Sony's optical (wireless) flash mode to signal a Neewer NW TT660 II flash, and in slave mode 1 the Neewer will trigger from the Sony body's flash and sync up to 1/160s.