The first distinction of the two systems is the way in which they trigger flash. The optical system relies on a line-of-sight connection between flash units. That is, for a unit to fire, it needs to be able to “see” another flash from the system. Some cameras allow the use of a built-in pop-up flash or another flash in the system to act as a master unit that doesn’t affect the exposure, but simply transmits a light pulse that triggers the slave units. There are a number of options for configuring an optical system, but you can’t get around the line-of-sight issue. The benefit of this optical configuration is that it’s relatively low cost and often easy to set up, but its limitations are in the line-of-sight restrictions and the occasional unpredictability of the system. The flash slave systems must be in the optical path of the master to be triggered, and the range at which you can move around is usually restricted to 40 feet or less.
On the other hand, we have radio-controlled transmitters/receivers to trigger strobes. The transmitter is connected to your D-SLR’s hot-shoe, while the receivers are connected to a power pack. With this setup, you can transmit a radio signal to your flash and quickly fire off shots with very consistent results and output. The radio triggers vary in their operational distance—anywhere from 100 to 1,600 feet—depending on the transmitter. The key benefits of a radio-controlled system are the increased range and the security of your strobe network. If you’re shooting a sports event, for example, you can position radio-controlled units all around and set them up so that no one else’s cameras will fire your network. One of the limitations of using a wireless radio transmitter is that many cameras’ TTL metering won’t work, and these units also tend to be somewhat more expensive than an optical system.
Most camera manufacturers have high-end flash units, and most of them can be used on master mode for slave unit shooting. Canon uses the E-TTL II system in which its Speedlites can be triggered by setting the camera to master so that the flash units can be simultaneously triggered. On the high end, the Canon Speedlite 580EX II and the new Speedlite 430EX II both can be remotely triggered, and the flash units are capable of automatically setting their own exposure through the lens.
Nikon’s Creative Lighting System sets its Speedlights to a commander mode as the master unit. For the D3, it uses an i-TTL-compatible hot-shoe, or if you’re shooting with a D300 or D700, you can trigger the Speedlights with the camera’s built-in flash as the commander. The Creative Lighting System consists of various Speedlights. The new SB-900 is a particularly noteworthy unit because it lets you control up to three remote Speedlight groups and an unlimited number of compatible Speedlights in its range.