An introduction to flash photography

A flash is a device used in photography producing a gleam of light to help illuminate a scene. Flashes are typically used to light subjects in fairly low light situations, to freeze motion of fast moving subjects, or to change the quality of available light.

There are 4 main types of flashes: the built-in flash which is fairly sufficient for average shots of subjects at close distances, flash guns or external hotshoe mounted flashes which are more powerful than built-in flashes and allow for light bounce for softer even illumination, hammer-head flashes which can be more powerful than flash guns and are mounted on a bridge that attaches to the bottom of the camera resting on the same level as it, and studio flashes which are used by professional portrait and beauty photographers for greater lighting power and are usually used with beauty dishes, softboxes, shoot through umbrella, and of the like attached to them.

In professional studio photography, flashes may be large, standalone units, or studio strobes, powered by special battery packs or connected to the main flash and synchronized with the camera from either a flash synchronization cable, radio transmitter, or are light-triggered by a master flash that is synchronized with the camera. These are called slave flashes and are used to provide fill light, rim light, and the like.

Flash guide number

A flash guide number evaluates its power and ability to illuminate a scene at a certain film speed or sensor sensitivity and angle of view. A higher flash guide number means a more powerful flash.

“The guide number is the product of the maximum flash-to-subject distance and the f-number of the aperture, that will correctly expose film or a digital sensor with the specified sensitivity.” ~ Wikipedia

The formula goes as follows:

GN = distance × f-number

Where GN is the flash guide number, and is the product of the distance between the photographed subject and the flash multiplied by the aperture or lens opening.

As you might have already concluded, if you double the power of the flash, you could either illuminate the subject at twice the previous distance or you could illuminate the subject at the same previous distance at 1/4th the previous film or sensor sensitivity.

Guide numbers could be given in feet or meters depending on the flash manufacturer, and are usually given based on an ISO 100 film speed or sensor sensitivity. Whatever the case may be, the previous formula still applies regardless.

“The guide number represents an exposure constant for a flash unit. For example, a guide number of 80 feet at ISO 100 means that a target 20 feet away will be correctly illuminated with an aperture of f/4 (80 = 20 × 4) using a sensitivity of ISO 100. For the same guide number and an aperture of f/8, the light source should be 10 feet from the subject (80 = 10 × 8).” ~ Wikipedia

You don’t really need to bother with all this, since light meters nowadays are built to point out all those settings for you, it is good to know nevertheless.

Flash sync speed

Flash sync speed is the synchronization of the flash firing with the camera’s set shutter speed. This info is important because if you set your shutter speed faster than your flash sync speed, you could end up with photos that are partly dark. That is because your shutter is too fast that it opens up to admit light to your film or sensor, and closes up before your flash has finished firing. Photo parts that did not make it to the flash will remain darker than those that actually did.

So the fastest shutter speed that you can use with your flash cannot go above your camera’s flash sync speed. Different camera models have different sync speeds, but most are 1/200th – 1/250th of a second. Refer to your camera manual to learn your flash sync speed.