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Light-actuated slaves are the smallest, least expensive, and most bulletproof of all slave units. With no moving parts, and, in most instances no power source or sync cords required, they are often encased in a hard clear resin and can be run over by a truck (but not a steamroller) and still work.
Sadly, they are also indiscriminate in terms of the light source that will trigger them and obviously, in a wedding scenario you don’t want the great, unwashed masses of point-and-shooters setting off your flashes. But (big BUT), if you are working in an arena where you are the only photographer (such as a studio or other single-photographer environment) light-actuated slaves are the simplest way to get two flash units to fire at the same time.
But do they really fire the slaved flash at the same time?
The first thing neophyte flash photographers question is how the slaved flash can fire at exactly the same time as the primary flash. In fact there is a slight delay between the primary flash firing and the slaved flash firing but in reality the delay is so short it’s almost impossible to measure. Still, newbies worry and fret about this, so let me put this question in its proper perspective.
Let's do the physics: A light-actuated slave closes its triggering circuit the instant it is hit by a high-intensity, short burst of light so there is no delay once the light actuates the slave. This means that the only factor in delaying the time at which a slaved flash fires is the distance between the primary and slaved flash. Now, suppose the primary flash and the slaved flash are 25 feet apart. Remember that light travels at 186,000 miles per second. You could quadruple the distance between the two flashes (or even set them almost a mile apart!) and it would still take less that 1/186,000 of a second for the light from the primary flash to reach the slave.
Because of the stupendous speed of light, for all practical purposes there is no delay between a primary and slaved flash’s firing (except maybe to a physicist interested in teeny-tiny numbers and equipped with multi-million-dollar testing equipment).
Lastly, you should know that while I carry four radio slave transmitters and six radio slave receivers in my equipment cases , there are also two or three light-actuated slaves tucked away in a corner of one case because…well, you never know!
The Dreaded Problem of Latching
One problem associated with all slaves is a situation called latching. This happens when the slave’s contacts close (which fires the flash) and then don’t reopen, so the flash unit keeps firing every time it recycles. Imagine a crazed firefly and you’ll get the idea. With light-actuated slaves the slaved flash will keep on firing along with it, until it either fries itself or pops a circuit breaker that can be reset.
I have found that latching often occurs in high ambient light situations such as the kind found when a slave’s receptor is near a window or under a strong incandescent light.
I stumbled on a solution to this problem as a happy accident, but you can benefit from my good fortune. As I unplugged and then re-plugged the latched slave (thinking it was a contact problem) the shadow of my hand momentarily covered the slave’s receptor and the slaved flash stopped its incessant firing. However, the second I moved my hand away from the slave the thing started firing the flash repeatedly once again.
I realized the shadow of my hand might have something to do with curing the problem, so I pulled out my handkerchief and draped it over the slave’s receptor. Voilá! The handkerchief worked; blessedly, the continuous flashing stopped! Knowing I might have to mop my brow (or blow my nose!) later in the day, I replaced the handkerchief with a piece of paper napkin taped over the slave. It worked too!
That night, after getting back to my studio, I could recreate the “latching incident” by shining a quartz light on the slave from a few feet away, and once again the piece of paper napkin solved the problem. I decided a more permanent solution might be a tiny piece of strapping tape (the kind with fiberglass strands running though it) and that worked too. This “slave with a strapping tape hat” became my favorite, most trusted light-actuated slave.
The following year a new, overzealous assistant removed the little square of strapping tape thinking he was being helpful by cleaning up my slave. Sure enough, as soon as the slave was plugged in, the flash started firing every time it recycled. Being on location, and having neither a handkerchief (well…I had one, but had used it!), nor a paper napkin, nor my trusty roll of strapping tape with me, I covered the slave’s receptor with a quarter-inch square piece of gaffer tape and, miracle of miracles, it worked too!
I can only surmise that the high level of ambient light was the culprit and the tape cut that down. Now, having read this tale, when you are faced with a latched slave, before you get so frustrated you start looking for a sledge hammer (or steamroller), try a quarter inch square piece of gaffer tape or a McDonald’s napkin instead…you might be pleasantly surprised by the results.
Here are four different light-actuated slaves that travel with me. Two of them (my first and second choices when I need one) are wearing gaffer tape squares over their sensor domes. Also note that one has my name on it because it’s easy to mix up who is the slave’s owner in multi-photographer environments and this is the one I lend out! Finally note that one slave has an patch of Velcro on it. This dates back to the days when radio slaves weren’t as reliable as they are today and I always stuck this slave onto my flash head so it would be ready for use if the radio stopped working (it can happen!).
Mounting a radio slave on a camera or light pole
I work within a community of between 30 and 50 photographers. In this community most of us use radio slaves, and those that don’t are the youngest photographers who are still saving their pennies to invest in a radio slave system. While mounting a radio transmitter module in a hot shoe on the camera can be done, almost all my co-workers and I mount our transmitters on our flash brackets instead.
Some use a screw-locking shoe mount made by Stroboframe bolted to their bracket while others have some form of metal plate covered with Velcro™ bolted to their flash bracket instead. The back of the radio slave transmitter is also covered in Velcro™ and the transmitter is attached with Velcro to the aforementioned plate on the bracket.
I use the Stroboframe shoe on my bracket but I even have a patch of Velcro™ on the side of my rollfilm SLR’s prism so I have a place to stick a transmitter when my camera isn’t mounted in my flash bracket. Likewise, all my AC flash generator packs not equipped with a built-in Pocket Wizard receiver also have a Velcro™ patch on their sides near the sync outlet so I can attach receivers to them easily.
I use Stroboframe’s flash shoe (it’s the piece with the red locking screw) screwed to my flash bracket to mount my radio transmitter. I’ve removed the radio-to-camera sync cable in this photo so you can better see the shoe, but if your camera is equipped with a hot shoe, you might want to mount the radio on the camera instead of on the bracket.
For attaching the radio slave receiver to a light pole, all of us use a plate of one sort or another covered with Velcro™. Mine (shown) is made from a piece of 1/8-inch aluminum and is slightly smaller than a cigarette pack. It is bolted to the light pole with flat head screws that were countersunk into the aluminum plate so the Velcro™ covering them (and the plate) presents a smooth surface for the receiver to Velcro to.
The screws pass through the light pole and are locked in place with nuts that have nylon inserts within them. The nylon inserts keep the screws from loosening. Look at the photos to understand how the whole thing fits together. It’s one thing to buy a radio slave system and another to figure out how to mount it.
It has been said that photographers are the kind of people who wear both belts and suspenders at the same time. In this instance, that means even though my radio receiver is securely fastened to my aluminum plate with Velcro™ I use PocketWizard’s little loop handle as an insurance policy. Photo equipment doesn’t bounce well off hardwood floors.