Most electronic flash units incorporate a slave cell, which 'sees' the light from another flash and fires the head instantaneously, so only one flash unit needs to be connected to the camera via a lead, but with some makes this is an optional extra.
With some makes the slave cell can be switched off - this in my view can be a nuisance because it is all too easy to switch it off accidentally and you may not notice that it hasn't flashed.
The flash that is fired by the camera should be one that is providing broad lighting - very directional lighting from a honeycomb or a spotlight may not trigger another flash. Sometimes the slave cell is badly sited (a classic case of the manufacturers never actually trying out their own products) and I had some units once where the slave could not 'read' a flash unless it directly caught the sensor which was right at the back of the head!
Firing the flash
The 'standard' method is a synch lead (also known as a PC cord). One end plugs into the flash and the other end goes to the camera, usually into a PC socket. Some cameras don't have a PC socket, in which case a PC adapter can be fitted into the hot shoe. However this is not ideal, especially for older flash units that have high trigger voltages which might damage the electronics in the camera. A sensible precaution, if you're not sure of the trigger voltage of your flash unit, is to protect the camera by fitting a Wein Safesync into the system - this controls the level of voltage reaching the camera. There is some interesting info about trigger voltages here.
A useful alternative to a synch lead, especially for any type of 'people' photography, is a means of triggering the flash remotely. Not only do synch leads get in the way, they are also often unreliable. Here are the alternatives:
1. An infrared flash trigger, which fits into the camera hotshoe and picks up from the hotshoe. They can also be plugged into the PC socket. They fire a signal to the flash slave (a bit like a TV remote control). These gadgets can be useful provided that you have at least one flash head that has a sensitive slave sensor, but they are not ideal for all cameras - when fitted to my RZ67 for example, the trigger gets in the way of the focus lock. They require a sensitive flash sensor on the flash head, and often don't work with older units. They are also fairly flimsy.
2. An ordinary on-camera accessory flash, fitted with a deep red lighting gel or bounced off the ceiling so that it doesn't add light to the shot. This works in the same way as the infrared flash trigger described above, and has the same limitations. The advantage is that you keep the £50 cost of the trigger where it belongs, in your back pocket.
3. Next up, in terms of both sophistication and cost, is an infrared flash trigger that sends its signal, not to the flash unit itself, but to a matched infrared receiver plugged into the flash. There are 2 advantages to this. They work equally well both with modern and older flash units. The other advantage is that they will only fire the flash if it is fitted with a receiver operating on the same frequency, so more than one photographer can work in a studio at the same time without setting off each other's flashes. See below for more detail on this.
Each of the above methods has a limited range (not usually important) and, in theory at least, requires line-of-sight to work. In practice however, the signals bounce around and so they are not too unreliable even when line of sight cannot be achieved.
4. Most professionals use a radio transmitter and receiver system. The transmitter plugs into the hotshoe or connects to the camera via a PC connection, and fires radio waves that are picked up by the receiver, which is connected to the flash. These systems don't require line of sight (in fact they will even operate through walls) and have a range of at least 100 yards. As I write, I'm looking at my own 'Pulsar' radio transceiver. It has 4 separate channels, so can fire up to 4 individual lights (provided that each has its own receiver) or can be set to fire all at once. by connecting it to one light and letting the flash from that light fire the other lights.
The advantage of individual firing is that it makes it very quick and easy to use the transmitter when metering individual lights. Any number of lights can be fired using just one transmitter and one receiver - simply leave the slave cells on each light switched on. The unit can also be set to any one of 6 different area channels, allowing up to 6 photographers to work in the same area without setting off each others lights.
Using optional-extra camera connections, some radio triggering systems can be used to remotely fire cameras too, but this type of sophistication doesn't come cheaply.
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Firing your studio flash without frying your camera
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