Most advanced SLRs as well as some intermediate point-and-shoots can be used with an external flash unit. Such cameras will have a flash shoe (often called a hot shoe), a PC-sync connector or built-in RF radio to communicate wirelessly with RF-equipped flash heads. If you do a lot of indoor photography (or outdoor photography where high contrast is a problem), consider an external flash unit.

Let’s back up a moment and look at the flash you may already have on your camera – the built-in one. These are very handy for a variety of situations but have two major limitations: positioning and light output. Point and shoot cameras tend to be quite small and so any built-in flash has to be quite close to the lens axis, which leads to problems like red-eye. Some camera designs put the flash head on an extension arm that pops up from the body, which improves quality somewhat by moving the flash head further away from the lens. However these innovations in flash location can’t solve the other problem, which is relatively low output.

The capability of a flash unit is usually stated as a Guide Number, which represents the maximum distance that the flash will be effective at a given ISO sensitivity and lens focal length. Some camera manufacturers simply state the effective distance range for various zoom lens settings in feet or metres. If the maximum range for your point and shoot is four metres, the flash isn’t going to be fully effective if you are more than that distance away from your subject.

External flash systems solve both problems at once. First they are bigger with larger battery packs and generate a lot more light than a built-in flash. If mounted on a camera equipped with a hot shoe, they move the flash source further away from the lens axis. As well, many on-camera flash units have a tilting and swivelling head, which allows you to aim the light source so that it isn’t pointing directly at your subject. Pointing the flash at the ceiling, for example, softens the light output and reduces the hard-edged shadows that are characteristic of unmodified flash lighting. Flash units may also come with light modifiers that also soften the light, or filters that change the colour of the light – useful for colour balancing as well as special effects.

Today external camera mounted flash units are electronic marvels, often integrating with a digital camera’s overall exposure control system. One of the most useful advances has been tying flash output to the camera’s through-the-lens or TTL metering system. Most major camera makers have TTL flash systems that go by a variety of names but operate in basically the same way. The camera’s exposure metering system exerts very fine control over the flash unit’s output for precise exposure. Before TTL flash you had to be something of a lighting scientist to apply the inverse square law to calculate the correct flash output setting.

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