Lighting The Way
Understand the fundamentals of flash to make the best purchasing decision
A flash's power rating comes in the form of a guide number (GN). The number doesn't reflect how far the flash will reach, but rather helps to calculate that distance. The formula (GN/ƒ-stop = distance) used to be a photographic "must-know" because flash exposures had to be calculated manually. Now the camera handles it automatically. An average built-in flash has a GN of around 43.
When comparing guide numbers of auxiliary flash units, it's important to note the flash's zoom setting. Some manufacturers figure the guide number with the zoom at a normal lens setting while others use a higher zoom position. For example, the Sunpak PZ40X has a GN of 105 at a zoom setting of 50mm, while the Sigma EF 500 DG Super sports a GN of 165 at a zoom setting of 105mm. Both units are quite capable of delivering light to a subject more than 30 feet away. So when comparing flash units, check the zoom setting to ensure a fair comparison.
In addition to offering more power, some models may include the ability to rotate and bounce the flash and greater coverage for ultra-wide-angle lenses. They also may feature wireless capability, high-speed sync and stroboscopic modes.
Through-the-lens metering (TTL) provides the means by which cameras achieve accurate flash exposures. Basically, the light emitted by the flash reflects off of the subject and back to the camera. The light passes through the lens and hits the built-in sensor. When the camera determines that sufficient light has hit the subject, it quenches the flash.