By James Booth
What the heck is a guide number and why is it important? I often wondered this myself. Had I taken the trouble to learn, a lot of my earlier attempts at flash photography would have been more successful. In essence, a guide number, or flash factor, is a measurement of light output for a given flash at a given film speed. In this article, I will attempt to be your guide to the guide number system.
"A guide number is a measurement of light output for a given flash at a given film speed"
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For each flash unit, the manufacturer determines the guide number by taking into account the light output, multiplication factor of the reflector, and ambient environmental reflective qualities. Generally, the published guide number for a flash is usually for use with 100 speed film. Because of the varying light sensitivities of the different film speeds, the guide number will change as the film speed changes. This is why it's important to hang on to those instruction manuals. Each of my flash manuals has a guide number chart for that flash's guide number by film speed. Figure A shows the guide number chart for my SunPak AP-52 flash.FIGURE A
Notice the guide number (GN) changes by film speed. (click for larger image)
This way please
So why is it important to know about guide numbers? Do you use a flash meter? If so, then you probably don't have much need for guide numbers. But if you want to take flash photos and don't have a flash meter, guide numbers can save you a lot of trial and error and go a long way toward making your photos come out better. Knowing about guide numbers can also help you in choosing a new flash or strobe by making sure you don't get one that's too weak for your needs. The higher the guide number, or GN, the more intense the light output.
By utilizing the guide number system you can better estimate the proper aperture setting, or f/stop, to use in order to get the best depth of field for your shot. So what's depth of field? Depth of field is the range of sharpness from the closest object in your shot, to the farthest. For more information on depth of field see my article "An in-depth guide to Depth of Field" in this month's issue of Connected Photographer at http://www.connectedphotographer.com/issues/issue200405/00001284001.html.
How will knowing a flash's guide number give you the appropriate aperture setting? If you divide the GN by the distance in feet the light travels to your subject, the resulting number will be the f/stop to use for the best depth of field. When figuring the distance the light travels, remember to take into account the distance from the flash to any reflective surface. For example, if you're using an umbrella reflector or bouncing off the ceiling, the measured distance is from the flash to the umbrella or ceiling, then from there to your subject.