Speedlights (Flash Units)
a Yongnuo YN-568Ex speedlight
Guide Number (GN)
The Guide Number is an indicator of how powerful a speedlight’s output can be. It was initially designed as a “standard” to express the longest distance a speedlight could properly illuminate a subject in correlation to the ISO/ASA (Sensor/Film sensitivity). You might see the Angle of View (which is associated with the lens Focal Length) mentioned too, since flash units have a mechanism/motor that allows them to “focus” the beam according to your focal length. Nevertheless, the Guide Number is calculated by the following formula:
GN = distance to subject * f/number (Aperture value)
For example if a flash has a GN100 at ISO 100, this means that used with a lens aperture set to f/2.8 it can properly illuminate a subject 35.7m away (100 = Distance * 2.8 => Distance = 100 : 2.8).
It cannot be used blindly as an indicator because every manufacturer uses a different ISO sensitivity and aperture for their calculations.
Lens Coverage (Angle of View coverage)
Each flash usually comes with a Lens Coverage list in its specifications. It simply states which focal lengths the unit can work with properly (i.e. how wide or how narrow it’s light beam can get). Most mid-end and high end flash units out there usually cover the range 12mm to 200mm. Using such a flash with a 11mm lens for example would give you a problematic exposure since there will be some vignetting on the corners of the exposure due to the flash beam being narrower than your angle of view. On the longer end of a telephoto lens, let’s say a 300mm telephoto lens, you will have an underexposed image since the flash beam won’t reach your target or it will not be powerful enough to illuminate it properly.
Recycle Time
The recycle time is the amount of time a speedlight needs to re-gain power after firing in order to be able to fire again. The recycle time varies mainly with the power output that you fire the flash at, the higher the output power you are using, the longer the recycle time gets.
A small (or decent amount if you are comparing very good ones to terrible ones) percentage of the recycle time depends on the batteries that you are using with the flash.
AF-Assist
the AF-Assist pattern projected by the Speedlight’s LEDs
Most speedlights come with 2 or 3 LEDs that once activated illuminate a red grid/cross pattern in front of it (hitting your “target”) to assist your camera in focusing in the dark/low light. It is a very useful feature, and if you are into dark room/club/events photography, you should make sure your flash unit or your wireless flash controller has an AF-Assist lamp.
