Sync terminal or socket

A cable connector for socket external flash units that’s still found on higher-end cameras like pro DSLRs but is becoming less and less common as photographers switch to wireless flash systems. These are usually triggered by a ‘master’ unit attached to the camera.

Speedlight or Speedlite

The names used by Nikon and Canon respectively for their camera flash units, both built-in pop-up flash and external flashguns. There’s nothing intrinsically different about these compared to regular flashguns – it’s just a different choice of name.

Slow flash or slow sync

Special flash mode where the camera’s exposure is extended beyond the brief burst of the flash. This makes it possible to record some of the ambient lighting too, and it’s a popular technique for illuminating a nearby subject brightly without losing background colour and detail.

Rear curtain flash

A special slow sync flash mode which fires the flash at the end of the exposure not the start. This gives more natural-looking results with moving subjects because any movement trail will be behind your subject and not ahead of it (which looks odd).

Pop up flash

Most cameras have a built-in flashgun which pops up automatically in low light or can be popped up by pressing a button. The flash can provide emergency light, but it’s harsh and short range. In many instances it’s best to leave the flash off and use higher ISO settings.

Guide Number (GN)

A measure of the power of a flashgun, whether it’s a built in flash or an external flashgun. You take the guide number and divide it by the subject distance in metres to get the lens aperture you should use. Flash power is usually controlled automatically these days, though, so the guide number is just an indication of the maximum power.

Forced flash

A mode where the flash is made to fire whether the light is low or not. Normally, the camera won’t fire the flash in bright light, but forced flash mode overrides this. Flash can be useful for fill-in light for portraits, even in daylight, and especially if your subject’s face is in shadow.

Flash sync speed

Digital SLRs and compact system cameras use focal plane shutters and these have a design limitation – there is a maximum speed at which the whole sensor is exposed at once. This is the maximum flash synchronisation speed. Beyond this, the sensor is exposed in a moving strip, which is no good for flash.

Flash output (manual)

Flash power is usually handled automatically by in-built flashguns and external flashguns. The exception is studio flash, where you adjust the power manually in fractions of full power. 1/1 is full power, 1/2 is half power, 1/4 is quarter power and so on. Some smaller flashguns offer manual power settings too.

Flash compensation

Flash power is normally handled automatically by the camera, but you can increase or reduce the flash power with the flash compensation option – it’s just like using the exposure compensation option with automatic exposure.


Most cameras come with a built-in flash, but it’s also possible to add a more powerful and versatile external flash to more advanced cameras. Flash illumination can be harsh and unpleasant, but in expert hands it can offer useful artificial illumination.