Most major DSLR Cameras manufacturers offer dedicated portable flash units that work interdependently with the camera's CPU, metering and focus sub-systems allowing the connected flash to automatically read the focus distance and set the flash units "angle of coverage" to match the focal-length of your lens. Additionally, The dedicated flash can calculate the "correct" amount and duration of light needed to produce properly exposed images with little intervention on the part of the user.

While dedicated Flash units extend the existing on-camera flash capabilities, they're basically still mounted directly above the camera lens. However, Often there are cabled extenders available which, when connected both to the camera and the flash, do offer the same auto-sensing functions as when it's attached to the camera physically.

Non-dedicated Flash units have been around for years and, can offer a more affordable alternative to the often more expensive name brand units. A note of caution on using flash equipment not designed for your camera, Check the Trigger Voltage output before attaching to your camera! Today's Digital cameras are susceptible to voltages higher than 6volts and can easily be damaged by older flash units if the TV is higher than manufacturers recommended limits.

Cabled

Remotely fire flashes using the physical connection between the camera hot-shoe and the flash

Optical/Infrared Slave

Accessory that connects to flash using either PC/sync outlet or, via physical connection to the base of the flash.

Radio Waves

Transmitter connects to camera via hot-shoe or via PC/Sync outlet (if available). Receiver connects to flash using either PC/Sync outlet, cabled cold-shoe or via physical connection to the flash base and/or PC input.

Flash Firing/Using a cable

Cabled

Dedicated Extensions are available for most dedicated flashes and are simply mounted in between the flash unit and the camera's hot shoe. Using this type of OCF technique is most likely the easiest way to add additional lighting capabilities to your camera system.

PC/Non-Dedicated Cables

Older, PC-sync style cables connect between the camera and the flash via PC male/female connectors or, by a molded cold-shoe/cable adapter. This type of connector sends a simple trigger signal to the flash when the camera shutter opens. No metering or adjustment of flash-output is available so, you'll need to manually set flash power output directly from the flash unit itself.

Flash Firing/Optical Slaves

Optical Slave

Optical slaves work by sensing light and triggering the flash they're connected to a micro-second after the originating flash. Often used for fill light or background light where the flash unit may be inaccessible. A variety of types, connection options available. Shown is an older Vivitar unit that offers connection both via hot-shoe and/or PC-sync.

• Upside: Cheap
• Downside: Limited, line of sight position required to trigger. 10-20' avg dist.

Infrared Slave

Similar in principal to Optical above but requires a transmitter mounted and fired from the camera hot shoe.

TIP* You can trigger many infrared slaves by taping a small piece of exposed (black) 35mm film and lightly taping it across your on-board camera flash. The film represses most of the visible light while allowing infrared wavelength to pass through and trip your infrared slave.

• Upside: Greater distance to trigger.
• Downside: not as commonly available as optical slaves.

Flash Firing/Radio Transmitted

Radio Transmitter

Work in "Pairs" of two or more usually a transmitter and a receiver or, for multiple flashes, multiple receivers. Transmitter mounts on the camera hot-shoe, receiver plugs into flash unit via micro-pc adapter or, cold-shoe micro-pc cable adapter.

Currently popular models: Pocket Wizards, Wein Micro Slaves, Quantum Freewire, and others.

• Upside: Reliable, Multi channel capable, up to 1500' distance
• Downside: expensive and, you always seem to need one more.

• Flash and Strobe Trigger Voltage Database
• Flickr Group (new) for Flash and Strobe Trigger Voltage reports