In situations where there is little or no light available, a portable electronic flash unit is an invaluable piece of photographic equipment. With fast films and long exposures, you may be able to shoot existing light pictures, providing your subject remains still long enough. Although you can certainly get better lighting control with elaborate photographic lights, the simplicity and portability of electronic flash is unbeatable.

Electronic flash provides an excellent source of artificial light for exposing black-and-white and color daylight-balanced film. Light from an electronic flash unit (strobe) is characterized by softness, short duration, and color balance, approximating that of daylight.

By measuring the amount of light that actually reaches an object or scene, you can obtain a numerical value that can be converted directly into a flash guide number. The numerical value is the light output rating of an electronic flash unit measured in beam candlepower-seconds (BCPS) or more correctly, effective candlepower-seconds (ECPS).

Every electronic flash unit is assigned a guide number as a measure of its light output or power. The higher the guide number, the greater the light output. Guide numbers for various film speeds are usually

provided with each electronic flash unit. Information packaged with film may also provide guide numbers appropriate to their speed in regard to the various powers of electronic flash units. Manufacturers tend to overrate the power of their electronic flash units. When guide numbers are assigned by the manufacturer, they base the guide number on an average reflective subject and in a room with 10-foot light-coloredceilings. By using these methods, the manufacturers are able to take advantage of the films exposure latitude.

Like exposure meters, guide numbers are not infallible and some variation from assigned values should be expected. To ensure accuracy of the flash unit, you must check the efficiency of your electronic flash unit to determine your own reliable guide numbers. The steps used to check efficiency are as follows:

1. Place your flash unit (on the camera) exactly 10 feet from a live model who is holding a series of cards-one for each f/stop marked on your lens.

2. Load the camera with the type of film you want to test.

3. Focus the camera on the model and make an exposure at each of the f/stops marked on the cards.

For each exposure, instruct the model to hold up the card marked with the f/stop to be used so it shows noticeably in the picture. Process your film normally, examine the proof sheet or slides carefully, and choose the one shot that best reproduces the model's skin tones. Multiply the f/stop on the card in that picture by 10 (the flash-to-subject distance) and you have the guide number for that particular film and flash unit combination. If, for example, the best exposure was made at f/8, the guide number is 80 (8 x 10 = 80). Once you have determined the correct guide numbers for use with various films, make up a reference chart and attach it to your flash unit.

Correct exposure with electronic flash depends upon four factors: The power or light output of the flash unit

The IS0 speed of the film being used

The flash-to-subject distance

The f/stop setting

Shutter speed is not a factor since the time of exposure is governed solely by the duration of the flash.

Notice we always speak of flush-to-subject distance, never camera-to-subject distance. With all types of artificial illumination (the same as with

sunlight), the only consideration is the amount of light reflected from the subject. The distance between the camera and the subject has no bearing on exposure. When the flash is used off of the camera, the basic f/stop is still calculated with the flash-to-subject distance.