It seems many people do not have an understanding of what guide numbers are, and how they are effectively used. A flash unit's guide number is used to determine the proper exposure when shooting manual flash without a flash meter. But with today's advanced flash systems, guide numbers are most often used to compare power output between flashes. Understanding how a guide number is used to create a flash image will help you determine what flash is right for your needs.
Specifically, a flash unitís guide number indicates how much light the unit will emit in relation to a standard film speed. The higher the guide number, the more powerful the flash. This number is usually indicated in the owner's manual of the flash. Itís represented as "GN 118 with ISO 100 film." Just try to keep one thing in mind: you don't necessarily need to buy a flash that reaches the other end of the block to photograph your friend thatís standing ten feet in front of you. You might give him bad sunburn with that big gun!
The other use of the guide number is to determine your correct aperture for flash exposure. It is important to know this when using manual exposure with a flash. A lot of the older flashes have some type of chart or scale to determine proper exposure. Of these, quite a few of them have large gaps in the distance scales or are just hard to read. Many newer flash units do not have these scales at all. This can be troublesome if youíre using one of the newer flashes from an automatic flash system on a manual camera.
Why use a new flash with your older camera? If you have to ask you haven't looked at new flash heads lately. Many new units are packed with features that will greatly expand the capabilities of your current camera. In many cases you don't have to upgrade your entire camera system to get the benefits of modern flash technology. Exactly what features will or won't work with your camera will vary depending on the camera/flash combination. Your best bet is to find a store that has the flash you're interested in and ask to see both the flash and the manual. Usually there is a chart in the flash manual that spells out feature compatibility on a camera-by-camera basis. Don't take the sales person's word that a particular flash "will work with any camera in the ABC line".
So once you find that new flash, how do you use it? To determine the correct aperture all you need to do is divide the camera to subject distance into the guide number.
Lets assume you have a flash with a guide number of 118 with ISO 100 film. You would focus on your subject and then read the distance on your lens that lines up with the focus mark. For the sake of this example let's assume your subject is twelve feet away. Divide your guide number by the distance and you would get your aperture.
Number / Distance
In this example you would get f/9.8 for an aperture. Since most of these numbers you would get will not be exact apertures, you simply set your aperture as close as you possibly can; in this case f8. The exposure latitude of the film will allow you to get a usable image if you are off slightly in your settings. If youíre still uncomfortable or want to be extra cautious, you can bracket your exposure. Just remember, the aperture is what controls the exposure with flash photography, so bracket with just the aperture, not the shutter speed, when using flash.
Another thing to keep in mind when using this procedure is when you are increasing your film speed, your guide numbers will double every two stops. This would mean the flash in this example would now have a guide number of 236 with ISO 400 film. This would give you an aperture of f/19.6. Going to an ISO 800 film, your guide number would jump to 472.
Don't want to do the math (it's OK to admit it)? The manual for most new flash units will contain a small chart or "cheat sheet" you can use.
It's important to remember guide numbers are a recommended starting point and you may need to tweak the exposures to your own personal taste. They will, however, give you good usable image to work with.
Hopefully this information will make it easier to understand the real differences in the power of the flashes, so you don't over or under buy. It's also useful to demystify using a flash manually if you want to use one of today's newer flash units with your older camera. Just remember the following simple formula and youíll be fine in the field.
Guide number / distance = f/stop