Gn (Guide number) This is a measurement of flash power, quoted in metres or feet, usually at ISO100. The maximum distance for a correct exposure equates to the Gn divided by the aperture. For example, a rating of Gn 24 would give a maximum shooting distance of 3m at an aperture of f/8.
This goes beyond a simple optical sensor for using off-camera flash in manual mode, and includes an infrared link for TTL metering communication. Some flashguns add RF (Radio Frequency) wireless communication.
A rear screen is fitted to all the flashguns on test, apart from the Nikon SB-500. The Metz flashguns incorporate touch-sensitive screens for the adjustment of settings.
You can typically apply changes to flash settings from menus in the host camera, but it’s often quicker to use the control panel on the flashgun.