C copyrighted 1999

There are times when nearly every photographer needs more light than is provided by the sun, roomlights or other sources. Some type of light is needed that is easily controlled, portable and economical to use. In years past, flashbulbs were the form of supplemental light most commonly used by photographers. Flashbulbs have been replaced by electronic flash as the type of artificial lighting most popular with amateur and professional photographers.

What is electronic flash and how does it work?
An electronic flash creates light by passing a short burst of high-voltage electricity between two electrodes in a sealed tube filled with inert gas. There are several components all electronic flash units have in common to produce light. A power source, whether household AC current or batteries, supplies electricity to a high-voltage storage capacitor. The capacitor accumulates the power and when the triggering circuit is closed, the energy surges to the flash tube. The inert gas in the flash tube becomes electrically conductive, causing the circuit to be complete and allowing the energy in the capacitor to pass between the two electrodes, producing an intense, short-duration burst of light. The capacitor is now drained and once again draws power from the batteries or AC current until it is full and ready to be triggered again.

Unlike flash bulbs, which can be used only once, an electronic flash is capable of providing thousands of flashes. An electronic flash also produces extremely short duration bursts of light, ranging from about 1/1,000 to 1/50,000 second in length. This makes electronic flash an excellent light source for stopping motion. The light produced by electronic flash units matches daylight-balanced color film, so special correction filters are not needed.

Manual & Automatic Electronic Flash Units
At first, every electronic flash unit was a manual unit, meaning everytime the flash is triggered, it produces the same maximum-level burst of light. Since the light output of a manual unit is a constant quantity, the camera's lens opening is adjusted to properly expose the film over a range of subject-to-camera distances.

A calculator dial on the back of the flash provides the photographer with the correct lens opening to use based on the film speed and the subject distance. When an automatic flash is triggered, a photocell measures the amount of flash illumination reflected back from the subject. When enough light for a properly exposed picture is reflected to the photocell, the quench tube diverts the remaining power in the capacitor from going to the flash tube. This stops the flash unit from emitting anymore light. The unused energy in the capacitor is "dumped" into the quench tube and the flash recycles for the next shot.

The photocell changes the amount of light produced by the flash unit when the subject distance changes so the film will be properly exposed without having to adjust the lens opening. Small, inexpensive flashes may have just one automatic range, (3 to 12 feet for example) while more sophisticated models offer a choice of several ranges.

An important advance in electronic flash design came with the development of thyristor circuitry, which prevents the unused energy in the capacitor from being wasted in the quench tube. This energy-efficient design reduces recycling time, particularly when the subject is close to the flash. Another advantage of thyristor circuitry is its vastly reduced power consumption since the capacitor doesn't have to completely recharge after each shot.

The latest improvement in electronic flash technology has been the development of "dedicated" flash units which are electronically mated for certain electronic 35mm cameras. Besides the single large synchronization contact on the hot shoe, dedicated flash units have one or more additional contacts that match corresponding hot shoe contacts on the cameras for which they are designed. When a dedicated flash unit is mounted in the hot shoe of a matching cameras and turned one, the camera's shutter speed will be automatically set to the correct synchronized speed. Some dedicated camera/flash combinations also set the correct lens opening, have a flash ready light in the camera viewfinder or operate with the camera's through-the-lens metering system. Dedicated flash/camera combinations don't necessarily take better pictures, but they help prevent wasted shots due to incorrectly made camera settings.

Dedicated flash/camera combinations have anywhere between two and five electrical contacts on the shoe. The contacts synchronize the flash and camera together, set the camera's shutter speed to the correct setting, activate the viewfinder ready light and perform other functions.

Three Types of Electronic Flash
Most electronic flash units fall into one of the three main design categories - shoe mount, handle mount and AC-powered studio flash equipment. The hot shoe mount is by far the most popular style of electronic flash. Ranging in size from ultra-compact models weighing a few ounces to large system flash units weighing around 21/2 pounds, all hot shoe mount flashes have a standard ISO mounting foot with an electrical contact that couples to the contact on the camera's hot shoe. Shutter cords are not needed when hot shoe mount flash units are mounted on cameras hot shoes, but a shutter cord is included with some flash units for the occasions when a flash is used off of the hot shoe. When a flash is mounted on a flash bracket, light stand, tripod or other support, the shutter cord is used to connect the flash to the camera's PC outlet for proper synchronization. Most hot shoe mount flash units are powered by "AA" alkaline or rechargeable nickel-cadmium batteries.

Handle mount electronic flash units are, for the most part, large, powerful units used primarily by professional photographers. Their size and weight dictates they be mounted to a camera with a bracket rather than the hot shoe. The batteries may be self-contained or in an external battery pack that the photographer carries with a shoulder strap.

The most powerful electronic flash available are studio flashes, which are powered only by AC electrical current. Studio flashes usually have a stationary power pack that is connected to one or more flash heads by cords, although some studio flash systems have the power pack and the flash heads together as an integral unit. Studio flash heads usually have built-in incandescent lamps that allow the photographer to preview the lighting effect that will be produced by the flash illumination. Unlike hot shoe and handle mount flash units which are mounted on the camera, studio flash equipment is mounted on lightstands. A shutter cord connects the power pack to the camera. Because most amateur photographers don't require the large amount of illumination produced by studio flash equipment, they are used primarily by professional photographers.

Power Sources
Most hot shoe mount electronic flash units are powered by "AA" size alkaline dry cell batteries. For the photographer who doesn't take a lot of flash pictures, alkaline batteries are an ideal power source since their power level doesn't diminish significantly with age. Many electronic flash units that accept alkaline batteries will also accept nickel-cadmium batteries which are charged in an external charger. Although ni-cad batteries are about four times as expensive as alkaline batteries, ni-cads are much cheaper to use in the long run because they can be charged up to 1,000 times. Ni-cads are best suited for photographers who use their flash equipment frequently and charge batteries before and after using them. Ni-cads do not store power as well as alkaline and should be charged before using for optimum performance.

Rechargeable lead-acid battery packs with belt clips or shoulder straps are available for many flashes. Quantum and other lead-acid batteries produce a large number of flashes per charge, rapid recycling and don't have a "memory" problem like ni-cads.

Many shoe mount and handle mount flash units have an outlet for an AC power cord. Running a flash off AC current offers the advantages of rapid, consistent recycling speed and economical operation. AC operation is ideal for situations where the flash doesn't need to be moved much, such as in portraiture, still life, table top, and other types of indoor photography.

Power Ratings
Flash manufacturers use guide numbers to rate the power of a flash unit. Small flashes have guide numbers ranging from 45-80, while large shoe-mount units will be up to around 130. Handle-mount professional flashes range from about 120 to 200. Guide number ratings are usually based on ISO 100 film as a standard. A guide number increases when high speed film is used and decreases with slower film.

Synchronizing Flash Units
A flash unit needs to be synchronized with a camera's shutter so the film will be uniformly exposed when the flash is triggered. By mounting a flash in the camera's hot shoe or connecting a flash's shutter cord to the camera PC outlet, proper connection will be made. With most focal plane shutters, the fastest speed that is synchronized with flash is 1/60th of a second, although it is faster on some models. Usually, the maximum synchronized speed is marked in a different color than the other speeds on the shutter speed dial. If you're not sure of the maximum flash-synchronized sped on your camera, check your instruction book. Flash cannot be used with the shutter set higher than the maximum synchronized speed because the shutter is partially closed when the flash is tripped, causing incomplete exposure of the image area. Electronic flash will synchronize with all shutter speeds slower than a camera's maximum synchronized speed. Cameras with leaf shutters synchronize with electronic flash at all shutter speeds.

There are times when a photographer may want to synchronize two or more flash units to create special lighting effects or to increase the total amount of light. This can be done in two ways. The easiest is to use a slave trigger, which trips the flash to which it is connected when the light from the main flash hits its photocell. A slave trigger is attached to each remote flash and fires the unit in proper synchronization with the main flash. The other way to synchronize two or more units together is with multiple outlet adapters, which allow several shutter cords to be plugged into the camera at the same time. A PC/PC extension cord can be used to position the flash unit farther away from the camera. It goes between the shutter cord plugged into the flash and the multiple outlet adapter or the camera's PC outlet.

Controlling Flash Illumination The quality of light produced by an electronic flash is satisfactory for most types of photography such as sports, photojournalism, candids and travel pictures plus ordinary snap shots. There are times, however, when the rather harsh light produced by an electronic flash needs to be softened for portraits, broadened to cover the area of a scene included in a wide-angle view and other situations where the subject matter requires special lighting.

If your flash has a short flash cord, you can move it farther away from the camera by using a PC/PC extension cord. The extension goes between the short cord and the camera

One of the simplest ways to soften and broaden electronic flash illumination is by bouncing it off a white ceiling or wall. Many hot shoe mount flashes are equipped with a tilting flash tube that enables the flash tube to be aimed at the ceiling while the flash is mounted on the hot shoe. Units that don't have a tilting flash tube can be used for bounce lighting by mounting them on a bounce bracket, tripod or light stand, or by holding the flash pointed towards the ceiling. In these cases, a shutter cord is used to connectl the flash to the camera. Unless an automatic flash has a tilting flash tube, it should be used on manual when bouncing light since the sensor won't be facing the subject.

Bouncing light off a white ceiling adds natural-looking dimension to pictures of people and greatly reduces dark shadows falling on backgrounds. Since bounced light strikes subjects at a high angle, it appears quite natural since we are accustomed to light coming from above. Bouncing light consumes more light than direct flash because of the increased distance and absorption of light by the ceiling. When bouncing light off average height white ceilings, open the lens about 2-3 f-stops to offset the light loss. Some bracketing of the exposure is still recommended for best results, however.

Another way to soften the light form an electronic flash is with an umbrella. Available in a range of sizes from 20 to 60 inches in diameter, umbrellas are also available with white, silver or gold fabrics. Silver fabric is the most efficient reflector of light and is well suited for general photography work.

Umbrellas are mounted on lightstands with the flash unit attached to the end of the umbrella shaft, pointed towards the umbrella center. A shutter cord and PC/PC extension cord are used to couple the flash and camera. Small umbrellas can be attached to camera brackets specially designed for this purpose. Umbrellas will require 1-3 f-stops compensation depending on size and fabric. This is due to the increased distance the light travels and because some light is absorbed by the umbrella. Automatic flash units with non-removable sensors should be used on "manual" mode when used with an umbrella.