Flash Slaves and Off-Camera Flash

by Ezra Waltermaurer

Posted: May 9, 2011

Placing a flash remotely and fired in sync with your exposure using a slave lets you place light exactly where you want it.

In a civilized world the concept of owning a slave is repulsive, but in the world of flash photography a slave opens up whole new possibilities. I am talking, of course, not about human slaves but rather an electronic slave – a little device that allows your flash to be positioned somewhere other than on your hot shoe and to be fired remotely.

When ambient light alone is not cooperating, or when we are trying to achieve a specific light scheme,  battery-operated flashes are an inexpensive and flexible way to add to and control the light. In my article Exploring the Benefits of the On-Camera Bounce Flash, we saw how pointing a flash mounted on the camera’s hot-shoe at a ceiling or wall can produce broad soft lighting. If there are walls or ceilings available it's a great technique, but sometimes we have nothing to bounce the light off of, and sometimes we'd like the light to come from somewhere else. This is when we take the flash off the camera and call in the slaves to help.

Invisible Wires

In the not-too-long-ago ancient days of flash photography, flashes and expensive studio strobe-heads were all connected to the camera through a sync cable. Found on just about every interchangeable lens film-camera but common today only on pro-level DSLRs, a small jack on the camera would accept the cable coming from the flash system and when the shutter was tripped it would send a signal through the cable to the flash telling it to fire, thus syncing the flash with the open shutter.

From the battery powered shaver to the cordless phone to home wi-fi networks, man has long sought to free himself from the cable, and photo engineers and inventors were right in step. Enter the slave – a device that does just what the sync cable did but without the cable.

Types of Slaves

On the left, a flash which uses signals from the camera in Nikon's wireless remote system to tell it when to fire and how much power to put out. In the center, a flash with manual power output settings attached to an optical slave via the flash's hot shoe. On the right, an inexpensive yet workable radio slave system that transmits a signal from the camera's hot shoe, telling flashes attached to the hot shoe connection of the receiver units when to fire

Optical slaves: These are among the very first slaves developed. Capable of being quite small, a light-sensitive circuit under a clear plastic covering can detect the light coming from another flash and cause the flash to which it is attached to fire at the same time as the key/main unit. Because light travels at 186,000 miles per second, there is essentially no delay between the firing of the two flashes. Of course, the initial flash has to be triggered by the camera first – and that may be because it is built-in to the camera, attached to the camera's hot shoe, though a cable, etc.

Optical slaves can be attached to flashes through special connectors or through the hot-shoe foot of the flash. Some flashes have an optical slave built-in. An optical slave needs to be able to 'see' the triggering flash so there are some limitations on where it can be placed in the lighting layout. Also, as an optical slave only tells the unit when to fire, you must use one of the automatic or manual modes on the flash itself to control how much light it is putting out.

Note: Most modern cameras with a built-in flash or with a dedicated accessory flash mounted to the hot shoe use something called a 'pre-flash.' This is a quick burst of light that the camera uses to judge how much power to give to the main flash used for exposure. This happens very quickly, but to the eye of an optical slave it is the pre-flash that might cause it to trigger-- too early to be part of the exposure.  Unless you are strictly using your flash on a manual power setting which doesn't use a pre-flash, make sure you are using an optical slave that is either dedicated or switchable to ignore the pre-flash so that it only triggers from the main burst of light.

• Infrared Slaves: These are similar to optical slaves in that it is light that triggers the slaved flash, but in this case it is infrared light. Infrared light is essentially invisible to the image sensor and can be used without contributing any extra light to the exposure. Like the optical system above, an infrared trigger unit must be directly connected to the camera by a cable or to the hot shoe and the infrared 'eye' must be connected to the flash. Infrared units also require the slave to see the trigger unit.

Some infrared transmitters and receivers are capable of transmitting exposure information to appropriately spec'd flashes – telling them exactly how much flash power to put out. Most of the major camera manufacturers have these systems built into their current lineups of flashes and in control modes in the camera – something that any interested student should explore by reading their manuals and joining manufacturer-specific forums.

• Radio Slaves: Just as your car antenna doesn't need to see the radio transmission tower directly to tune in a station, a radio slave can receive triggering signals with no line-of-sight view necessary to the transmitter. Radio waves can go just about anywhere – through walls, around corners, across great distances - radio slaves allow for the most flexibility in lighting placement. Some systems are 'dumb' in that they only transmit the signal to fire, while others are specifically designed to tell the flash exactly how much power to pump out based on settings made in camera menus.

Although there are some decent inexpensive units, radio slaves that are extremely dependable and relied on by working pros can often be several hundred dollar investments. 

A single remotely fired slaved flash is placed behind the subjects and fired at a white wall. With a shutter speed of 1/200 sec and an aperture of f/8, the ambient light of the room was not bright enough to be recorded on the subjects' faces allowing for a sharp double silhouette.

Free Your Flash with a Slave

Once you've set yourself up with a slave-operated flash or two you can play with arranging your off-camera light sources. There are numerous stands and clamps available fairly inexpensively that will allow you to mount your flash(es) on sturdy supports and freely move them around. Just to throw out some ideas, you could try putting a flash behind your subject to offer some subtle or dramatic back lighting. Sending a flash up a tripod and having it face your subject from above and to the side is the basis for classic portrait lighting. Placing a flash low and pointing toward a wall behind your portrait subject can make an otherwise plain wall light up with some graduated tones and make for a pleasing background.

As mentioned in my article Understanding Basic Flash Photography, you will need to take control of the light output of the flash(es) to get the look you are after. You may be using an automatic setting as controlled by your camera and/or on the flash or you may be setting the power output of each flash manually – either way you'll need to play around with your exposure to dial-in your lighting, and if you are using more than one flash, you'll need to concentrate on the ratio of light between the two or more units. To pick from the suggestions above, you may want to have the key light (the light above and slightly to either side of your subject) as the main source of light but throttle down the backlight so that there is just a slight glow coming from behind your subject.

Learning about photography is all about experimenting. Flash and multiple-flash photography in particular can seem daunting at first. Get out your equipment, read up on it, set it up, take some shots, make some mistakes, make some mental notes, get some happy accidents and soon you will be on your way to understanding and enjoying all the possibilities of setting your flashes free with a slave.

Especially useful at fast moving parties and events, the Two-Armed Bandit technique allows you to position yourself quickly and provide pleasing light from angles you could never get with a camera-mounted flash. It's all about flattering your subjects with a gentle modeling light and creating plenty of separation from distracting party backgrounds.

Try This: This is a very spontaneous technique I call the Two-Armed Bandit. It's a great way to photograph a party or event where there is some ambient light but you want to add some motion-stopping directional flash to your subjects. The Two-Armed Bandit consists of your camera in one hand and a slaved flash in your other hand. I either use Nikon's i-TTL wireless slave system or an inexpensive radio slave to do this. The camera is held to your eye with one hand taking care of AF and shutter duties while your other hand is free to swing the flash in an arc around to light your subject from different angles. You can hold the flash high and to the left for a pleasing portrait light, choose to turn the light toward a wall or ceiling for a diffuse bounce flash or try some more dramatic side-lighting. This freedom of movement and flash positioning really gives you quick access to major and minor adjustments in light direction and will give you speedy feedback on which angles work and which are less successful to your eye.