Many DSLRs have a built-in flash for when the camera detects that there is not enough light for a well-exposed picture. Although the built in flash units are more powerful than you usually find on a typical digital camera they are still somewhat limited in range.
To overcome those limitations you can buy a flash gun to attach to your Digital SLR. A flash gun connects to your camera through the hot shoe on the top of the camera. These external units are far more powerful than the built in flash units.
Depending on the flash unit you buy you can gain far greater control of how you use flash. For example a built in flash unit is in a fixed position and will fire directly at your subject. With some external flash units you can move the head to fire at angle. One use of this is if you are taking a portrait shot you can bounce the light off a ceiling onto your subject. As well as giving you a more subtle light you radically reduce any possibility of red eye.
When buying a flash unit there are a number of points to take into consideration. The key considerations are described below.
Firstly, the built-in flash will not be very powerful. An indication of the power of a flash can be found in the 'Guide Number'. In simple terms the guide number gives an indication of the maximum distance, in metres, that the flash will be able to illuminate. The pop-up flash units in most DSLRs have a guide number of around 12 or 13. If you buy a separate flashgun you will find there are models with guide numbers as high as 30, 40 or even 60.
Another problem with built-in flash is that the flash head is too close to the lens, so if you are shooting people and they are looking directly at the camera you get the dreaded red-eye, caused by the light of the flash reflecting off the back of your subject's eyes. This is almost impossible to avoid, but most DSLRs have a red-eye reduction facility, which may help.
If you are going to invest in a separate flashgun it is a good idea to check the fastest synchronisation speed. If too fast a shutter speed is used it can mean the exposure is completed before the flash has fully fired, and only part of the scene will be lit. The mechanism that controls the synchronisation of flash and shutter speed to ensure the flash fully fires whilst the shutter is open is known as x-sync. Modern DSLRs typically have a fastest sync speed of around 1/200th or 1/250th of a second, but some can sync at faster speeds.
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This guide was written by Ian Younger
Using flash with slower speeds can produce some very interesting results when photographing moving subjects, depending on exactly at what point during the exposure the flash fires. By default a flash will normally fire at the very beginning of the exposure, which is known as first-curtain or front-curtain firing. This can produce odd results, though. For example, if you shoot a moving car, the initial flash will produce a sharp, 'frozen' image of the car but the movement of the car during the remainder of the exposure will produce motion blur streaks. The streaks will be in front of the car, which will create an impression that the car is moving backwards.
If you want the motion streaks to appear behind the car, to give a more natural result you need to be able to tell the flash to fire at the end of the exposure - second-curtain or rear-curtain flash. Not all cameras can do this, so it is worth checking if you are likely to be using flash with slow shutter speeds and moving subjects.
As well as offering a fast sync option, some models allow a slow sync flash seting. This is ideal in situations where you want to illuminate a foreground subject but also have the background correctly exposed. A fast shutter speed with a burst of flash does not allow enough of the ambient light to be gathered to correctly expose the background, so using a slower shutter speed solves the problem. Ideal if you want to take a picture of a person at night with a floodlit building in the background, for example.
In addition to attaching a flash gun to your Digital SLR's hot shoe on the top of the camera body, you can also use a flash gun "off camera". When using a flash unit off camera you would normally attach the flash unit to either a tripod or a lighting stand. The advantage of using off camera flash is that by moving the flash unit away from the camera you can change the angle of the light. This can create a completely different effect on your photos compared to on camera flash. This allows you to be far more creative.
There is nothing particularly tricky about off camera flash. You buy a trigger. The trigger has two parts. A transmitter is attached to your camera's hot shoe. The receiver is attached to the flash unit. When you take a picture the transmitter transmits and the receiver receives and your flash gun fires! That's it.
Depending on the quality of the trigger you buy you may have to manually set your exposure details for the flash unit. More expensive triggers can transmit exposure information from the camera to the flash unit.
As with camera lenses there are third party options available as well as flash guns made by the company that makes your camera. By all means shop around and find a flash unit that suits your requirements and your budget, but always make sure any flash gun is compatible with your camera.
As long as you can find a flash gun in good condition you can save a worthwhile sum of money by buying a used flash gun. The same compatibility rules apply as when buying new. In addition you will need to satisfy yourself that the unit has been well looked after and is in full working order. Whenever you can, ask for a guarantee when buying used camera equipment.