Whether you can add an external flash to your camera depends in what provisions the manufacturer made for them. Many point & shoot as well as DSLRs cameras have a “hot shoe” designed to connect a flash. In other cases there may be a connector built into the camera for a remote flash unit. The pc connector is the most often used type, however some manufacturers choose to provide their own connector, often because they need communication between the camera & the flash unit, something that pc connectors don’t provide. In the worst case, there will be no connection for an external flash – you will be limited to the camera’s flash.
Like most camera accessories, add-on flash units come in a wide range of qualities & capabilities. The least expensive versions simply provide an additional flash for your camera. The most expensive, particularly when designed to work with your brand & model, can communicate with the camera, automatically reading the camera’s aperture setting and adjusting the amount of light produced, or even reading the amount bouncing off the camera’s sensor during exposure. In some cases, the spread of the flash will adjust to match the setting of your zoom lens!
One of the first reasons for wanting an external unit is to increase the amount of light available when taking flash pictures. Because your flash produces a cone of light, the amount hitting the subject falls off at the inverse square, that is when you double the distance you only have one fourth the amount of light. It is typical for the flash unit built into cameras, either point & shoot or DSLRs, to be capable of working with a normal or telephoto lens up to a distance of 10 – 15 feet, but much over that distance requires an external unit.
Flash units are rated by a term called a guide number. They are usually given for ISO 100, and in the days of manual cameras, were used to determine the correct f: stop to use to provide the correct exposure when you know the camera to subject distance. Although you can still use the flash & your camera in the manual mode & do the math, because most modern external flash units can be set in an “automatic” mode. The flash head reads the amount of light reflected from the subject, and, as long as the camera or photographer tells the flash the ISO setting & camera aperture, the photographer no longer has to do those calculations. Generally, ISO numbers are now used to determine the maximum output of the flash. The higher the number, the more light the flash unit is capable of producing. One caveat – be sure the flash unit covers the width of your lens. If you are working with very wide angle lenses, it is likely the beam of light from the flash head will be narrower than your lens. Add on lenses, frosts or diffusers can be used to spread out the light from the flash head, however the wider you spread the light the smaller your guide number. Because of this be careful of manufacturers of variable spread flashes that advertise their guide number at the narrow setting. The number will be less at normal spreads.
Another reason for an external flash is to provide the benefits of bounced flash. One of the problems of direct flash is the harsh shadows & unnatural lighting it produces. An other is “red eye” caused by the flash source being too close to the camera lens. Many external flash heads can be rotated so that they are aimed at the ceiling. This indirect light will often look more natural.
Some external flash units can be “slaved” so that they don’t need to be attached to the camera. This allows the flash unit to be placed at a distance from the camera, or lets the photographer work with multiple external flashes. Some manufacturers build this capability into their camera system, or it can be added with additional radio or infrared links. Although there are also simple devices that detect a flash going off & then trigger a remote flash, they need to be used with care. Modern camera flashes, including external ones, often use “preflashes” to determine the correct exposure. These fire at low power a few milliseconds before the main flash goes off, but may be powerful enough to fire the remote. Since a typical strobe flash only lasts a small fraction of a second, and the shutter is only opened for a fraction as well, the flash may not be synchronized with the shutter, so even though the external flash is triggered, its light never reaches the camera’s sensor. This can be frustrating to the user because even though the remote flash fires, the resulting photograph is dark. Some manufacturers provide internal flash settings that shut off preflashes – check your manual. Again, it is important when using a remote wireless flash head that the camera & flash are capable of communicating with each other. This is a good reason for purchasing the flash & camera from the same manufacturer.
One last warning – don’t dig out that old strobe from your film days & plug it into the pc connector on a modern digital camera. Since the older cameras used actual mechanical switch contacts the voltage of the strobe’s firing circuit could be very high, in some cases a couple of hundred volts. Since the current was very low, this didn’t cause a problem for mechanical contacts, however modern cameras use electronics rather than a mechanical switch. The high voltage produced by older strobes can damage the electronics of a modern digital camera. Although an isolator such as the Wein Safe Sync can be used, unless you are firing expensive studio strobes, the advantages of modern external flash units make them well worth updating your equipment.
For some suggestions on how to use an external flash as well as one built into a camera, check my earlier article “Improving Your Flash Photography.”