Flash

Guide number

The guide number is a measure of the maximum distance from the subject it is possible to get a correct exposure. It is not comparable between different models unless it is measured in the same way on both. On modern flash units the guide number changes depending on the angle of view of the lens on the camera because the flash unit can focus the light to match the angle of view of the lens. This gives higher efficiency and longer range.

Before the guide number can make any sense it must be known if it is in feet or meter, what ISO and what angle of view is covered.

To find the range you divide the guide number with the aperture value of the lens. The Nikon SB-600 has a guide number of 30 m at ISO 100. That means that with a aperture value of 16 the range is 30/16 = 1.875 m at ISO 100. If you double the ISO the range will be multiplied with the square root of 2 which is approximately 1.4.

Recycle time

This is the time the flash need to recharge after a full power flash. It is not comparable between different models. A long recycle time can mean that the charging circuit is weak but it can also mean that the flash is very powerful. So the recycle time alone says very little about the flash unit. If the recycle time is long because of a large energy storage capacitor the flash unit will be able to fire many times at a rapid rate at normal power before it needs to be reharged.

Battery life

Battery life is often listed as the number of full power flashes, in this form it is not comparable between different models. A low number can mean that the charging circuit is inefficient and wasteful of energy but it can also mean that the flash is very powerful. In normal use the flash will use only as much energy as needed and a flash unit listed with a low number of full power flashes may well have a longer battery life than a flash unit listed with a high number of full power flashes.

Wireless operation

Some cameras can transmit information to an external flash unit by flashing the internal camera flash. Some require a separate transmitter mounted on the camera. Wireless operation makes creative lighting much easier.

High speed synchronization

The shutter in a camera has a minimum operation time. If the exposure time is shorter than this time the flash can't fire because the shutter will shade for the light in some of the image. Some flash units have high speed synchronization that makes the flash able to fire repeatedly during the whole operation time of the shutter to get an even exposure of the whole image.