Flash is a powerful tool in imaging.  Flash allows you to bring more light to a subject and to freeze action that is too fast for even the camera's shutter.  Whether built in to the camera, attached to the hot shoe, or connected by wires or a wireless link, mastering flash will help make you a better photographer.

 

 

A flash unit consists of a small xenon filled tube and the electronics to supply the tube with a brief jolt of high-voltage electricity.  Flash units are configured in a number of different ways:

 

On camera flash

These units (red arrows) are built right into the camera.  This is an advantage in that they are always handy, but:
  1. They increase the weight of the camera.
  2. They are underpowered.
  3. They draw their energy from the expensive camera battery, and can drain it quickly.
  4. Because they are close to the lens, they may cause "red-eye" in the picture.
Hot-shoe mount flash

Bigger and heavier, these units contain their own batteries (typically inexpensive AA cells).  They have more features, they are less prone to red-eye (since they are further from the lens), and they can often be mounted on extension cords or used with a wireless control system for even more flexibility. This unit also has an infrared light source (red panel) that helps the camera's autofocus system under dim conditions.
Macro ring flash

These units fit the flash head in a ring which mounts around the lens of the camera.  They can get the light on even close-in subjects, but don't work well if the subject is more than a few feet away - and talk about red-eye!  The battery pack of this unit (left) fits on the camera's hot shoe.

Whatever the type of flash you get, one thing to keep in mind is the Guide Number This is a number, usually expressed in feet at a given ISO, which indicates the relative power of the flash.  A higher number means a more powerful flash.  For instance, two flash units may have guide numbers of 50 and 100 with ISO 200 film.  This would mean that the former is powerful enough (with a normal lens) to illuminate a target 50 feet away.  The second unit is more powerful, since it would reach 100 feet.

Most flashes are designed to work at shutter speeds from 1/60 to 1/125 of a second, this is called the sync speed.  Newer cameras have a wider range.  On the other hand, the shutter speed is somewhat irrelevant in situations where the flash will be the main light.  Under these conditions, the brief (thousandths of a second) flash burst will freeze motion.

Other factors affect the useful range at which a flash will work.  A wide-angle lens requires more light than a normal lens, for instance, because at a given range a larger area must be lit.  Some cameras and flash communicate with each other; the hot-shoe flash shown above can find out from the camera what lens is mounted and then zoom the flash head (automatically, via a motor) to provide the right angle of flash to complement the lens.  Multipliers, extension tubes and other accessories may also affect flash exposure.

Flash used to be a tricky business, but electronics has changed all that.  Newer cameras either read the flash exposure by measuring the light as it is reflected off the film itself during exposure, or they fire a pre-flash and use special metering cells to determine the proper flash exposure.  In the former case, the camera simply shuts off the flash when the right amount of light has reached the film.  In the latter, the camera simply fires the flash for the correct amount of time.  Most cameras can easily do "fill-flash" as well. In fill flash, the camera makes an exposure using the natural light that is very close to correct, and fires the flash for a very brief time.  This brief burst of flash "fills" in the shadows and brightens colors.  Because the exposure was largely made using ambient light, however, objects in the background appear normal, not black as can happen when flash is used as the main light.

If you are using flash in any of the modes mentioned above, remember that large objects or objects close to the flash will reflect a lot of light back at the flash.  If they are large in the field of view, they can adversely affect the exposure, if they are small they will likely be over or underexposed themselves.  For instance, a blade of grass in the foreground of a macro shot may become completely white in the image, or the camera may use so much flash trying to turn a black dog into a gray one that the rest of the picture is overexposed.

Some hints on flash:

1.  Use extension cords or wireless remote units to get the flash away from the camera.
2.  If the flash is too harsh (particularly with close subjects), use a softbox or place a tissue over the flash head.
3.  Many flash units have a bounce capability, which allows you to swivel the flash head and bounce the light off a ceiling or wall.  This results in a softer, more natural light.
This only works if the ceiling or wall is close.
This requires a more powerful flash.
Don't try this with a colored ceiling or wall!

If there is no ceiling nearby, another accessory, a pocket bouncer, will fit on the flash and serve as a surface for bouncing.

4.  Beware of shadows cast by objects caught in the flash light.
5.  Fresher batteries will allow the flash to recycle faster.  Also, the flash will recycle faster when the subject is light colored and close.  Dark, distant objects will use up all the flash's power and cause a longer recycling time.  Pressing the shutter before the flash has recycled may result in a blurred or dark shot.
6.  Many flash units have a test button and a flash confirmation light.  Firing the test button will produce a flash; if there is enough light the confirmation light (usually green) will come on.  This lets you know if you have enough light before you waste film.
7.  On overcast or rainy days, fill-flash can bring out colors.
8.  Some flash units have a slave feature.  This means that they will fire when they detect a flash from another unit.  This is handy in two cases:
  1. You are a victim of a media circus.  Hold your slave unit in front of you and it will fire a blinding flash into the camera of anyone who tries to take a flash picture of you.
  2. You want to add additional light, perhaps from a flash unit closer to the subject, but don't want to string wires all over the place.
In the second case, an accessory even more useful than the slave is shown in the image above left.  The TTL slave sensor works with automatic cameras and flashes.  Not only can it sense when the flash is firing (and trip its own unit), it can also sense when the camera has sensed enough light and is turning the main flash down.  At this point the TTL slave unit shuts off its own flash.  The result - automatic, wireless remote flash exposures.  Some camera manufacturers are now building this feature into their own flashes.  I use the add on unit, shown here velcroed onto the top of my flash unit.  When coupled together by an off-camera extension cord, this makes for a more compact rig than you get when simply attaching the flash to the built-in hot shoe on the slave sensor.  With my macro rig set up on a tripod, I can hold the flash/slave unit at the precise position to augment the light from the ring flash.  I use an remote control or the camera's self timer to trip the shutter. 
9.  A Flash Extender can allow you to use a flash with a telephoto lens at greater distances than the flash would normally reach.  Flash extenders focus the flash light.