A flash is a separate device used in photography that is used to provide a flash of artificial light (typically in the range of  1/200 to 1/1000 of a second) at a color temperature of circa 5500 K to assist in the illumination of a scene.

One the primary purposes of a flash is to illuminate a dark scene. A flash can also be used to ‘freeze’ quickly moving objects or changing the quality of light. The term ‘flash’ refers either to the flash of light itself or to the physical electronic flash unit that discharges the light. Most modern flash units are of the electronic type, having evolved from single-use flashbulbs and flammable powders. Most of today’s modern cameras incorporate integral flash units, all bar the more advanced professional types.

SLR cameras allow separate flash units to be mounted via a standardised “accessory mount” brackets (hot shoes). In professional studio equipement, much larger self powered stand-alone flash units, or studio strobes, powered by special battery packs or connected to the mains, are synchronized with the camera from either a flash sync cable, a radio transmitter, or are light-triggered, meaning that only one flash unit needs to be synchronized with the camera (known as master), which in turn triggers the other units (known as slaves).

Most modern flash heads have LCD rear displays & also incorporate adjustable heads which can be tilted & rotated to suit the application.

A problem with cameras using built-in flash units is the relative low intensity of the flash output. The light level will often not suffice for good pictures at distances greater than 3 meters (10 ft). The resulting images are fairly dark pictures with quite excessive noise or “grain”. In order to get good flash pictures with simple cameras, it is important not to exceed the recommended distance for flash pictures.

The “Red-eye effect” is also another common issue. The retina of the human eye reflects red light straight back in the direction it came from, pictures taken from straight in front of a face often exhibit this effect. It can be somewhat reduced by using the “red eye reduction” found on many cameras (a small singular or several very short pre-flash bursts that contracts the subject’s iris). Alternatively, very good results can also be achieved with a flash unit that is detached from the camera of a sufficient distance away from the “optical axis i.e. not directly in line with the lens & subject (as it would be if the flah is mounted on the camera’s hotshoe), or by using “bounce flash” where the flash head is angled away to a nearby reflective surface namely a wall or ceiling. The flash ‘light’ is bounced back from the surface & hits the subject from all angles thus eliminating red eye completely. Bouncing flash onto your subject also creates a much softer, indirect & even illumination of your subject. Note: when bouncing flash light it is crucial to choose a white or very near to white surface to bounce off else a colour wash will saturate your image the same as the colour of the surface that the light bounced off.

Some in-built flashes fire a pre-flash immediately before the main flash. In some camera/subject combinations, this will lead to shut eyes in every picture taken. The blink response time seems to be circa 1/10 of a second. If the exposure flash is fired at approximately this interval after the TTL measuring flash, people will be squinting or have their eyes shut. One solution may be the FEL (flash exposure lock) offered on some more expensive cameras, which allows the photographer to fire the measuring flash at some earlier time, long (many seconds) before taking the real picture. Unfortunately many camera manufacturers do not make the TTL pre-flash interval configurable.

In summary, flash photography is an art in itself but not such an art that is difficult to understand & get to grips with. I highly suggest you stretch your budget as much as possible & purchase the best flash unit you can buy but make sure you stick with one that is made by by the same manufacturer as your camera.