A good flashgun is one of the most essential and versatile DSLR accessories. It\'s not just for dark times, when you\'re shooting indoors or at night - a flashgun is equally useful for filling in unsightly shadows in bright, sunny-day portraits.Unlike the pop-up flashes in most Nikon DSLRs, flashguns give you more power, greater flexibility over lighting techniques and, in some cases, advanced facilities for wireless multi-flashgun shooting.The maximum power of a flashgun is indicated by its guide number (GN). This is usually stated for shooting at a sensitivity of ISO 100 and a focal length of 105mm. That\'s because most flashguns have motorised zoom heads that automatically adjust as you alter the zoom setting of your lens, or fit prime lenses of varying focal lengths.As you stretch from wide-angle to more telephoto focal lengths, the light used to illuminate the periphery of a scene is wasted. So, by zooming the flash head, its light is concentrated on the area that will appear in the photo. The GN enables you to know the maximum range of the flashgun at any given aperture. You simply divide the GN by the aperture you\'re using.As an example, a flashgun with a GN of 40 would enable you to shoot an object from up to 10 metres away with an aperture of f/4, or from up to five metres away with an aperture of f/8. That might sound like more power than you\'d ever need, but there are other factors to take into account.Directing flash.The GN gives maximum distances only when you\'re aiming the flashgun directly at a target. However, one of the most essential features of any good flashgun is a bounce and swivel head. Due to the small physical size of a flash head, direct flash produces \'hard\' lighting that can be unflattering for portraits, and cause dark shadows. By tilting the flash head upwards in indoor portraiture, you can bounce the flash off a white ceiling. This effectively gives a much larger source of light, which makes for softer lighting.The downsides of this are that the distance between the flashgun and target is increased and not all of the light is reflected, so flashguns with extra maximum power come into their own. The swivel facility does the same job when you\'re shooting in portrait orientation.Working out the manual flash setting, especially when bouncing flash off walls or ceilings, can be a nightmare. Thankfully, all the flashguns in this test are fully compatible with Nikon i-TTL (intelligent Through The Lens) flash metering. This aims to ensure accurate and consistent flash power for correctly exposed images in any conditions.A practically imperceptible burst of pre-flashes is fired to work out the correct flash exposure for the scene, just before the camera\'s shutter opens and the shot is taken. That\'s the theory, anyway, although we found the accuracy of i-TTL metering varies with different flashguns in our tests.The zoom range of most flashguns is about 24-105mm, but this is for full-frame cameras such as the Nikon D700. However, DSLRs such as the Nikon D3100, D5100 and D7000 and D300S have a smaller APS-C sensor. With the 1.5x crop factor of these cameras, the effective zoom range translates to 16-70mm.