You have the camera, the strobes or flashguns.
There are two main types of external flash units that can be connected to the camera itself via its "hotshoe" (the metal bracket on the top of your camera): a strobe version.
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What Is GN?
Guide numbers, better known as GN, are the units used to measure the power of a flash (usually 1001.html'>Understanding Guide Numbers.
When Buying Look For...
* '''Compatible trigger voltage''': if it is too high it will damage your camera.
* '''Autoexposure, Thyristor Control, & Shutter speed auto-settings''': in automatic flash units this is what corrects flash metering.
* '''Zoom''': necessary for controlling light levels when using a zoom lens.
* '''Flash''': ready light.
* '''Bounce capability''': a flash unit with a base that can swivel or twist.
* '''Autofocus units''' for AF cameras.