BetterPhoto Q&A
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Esther Mishkowitz		understanding flash apertures i understand f stops on the camera body and how changing the number allows different amounts of light in -in conjunction with the shutter speed, however, I dont understand how the f stop on the flash only (not the camera (f stop) works in conjunction with the flash. with the flash on manual . am I shooting more light at 5.6 on the flash than at f11 or is there more light shot from the flash at f11? how does it affect in relation to distance would I change the f stop if the subject is further or only if I want to add more light on any object within a certain distance (on the scale) I hope I am being clear. and thank you. I dont think ive ever seen this issue discussed. To love this question, log in above November 13, 2003

Jon Close		Hard to give specifics without knowing the make/model of your flash. More generally... The flash is of very short duration, from 1/500 to 1/10,000 of a second. So varying the shutter speed from bulb, 30 seconds, down to the camera's flash sync speed (commonly 1/60, or 1/90, or 1/125, or 1/200) has no effect on the amount flash exposure. Changing the aperture (on the camera/lens) will control the amount of flash light in the exposure. The intensity of the light drops off with the square of the distance from the flash to the object being illuminated. A subject 20 ft away receives only 1/4 the amount of light as a subject 10 ft away, and only 1/16 the light of a subject 5 ft away. So for a flash firing full power on manual, you use smaller apertures for subjects close to the flash to keep them from being overexposed, and larger apertures for more distant subjects. With the flash firing full power on manual, the exposure is determined with guide numbers. The flash has a rated guide number stated in feet or meters and usually for ISO 100 film. The correct aperture number to set is calculated as: f-number = (GN / D) x Square Root of (film ISO / 100) / D where GN = Guide Number in feet or meters D = subject to flash distance (in the same units as the GN). The guide number calculation is not necessary with automatic flashes. These use a built-in light sensor, or the camera's own TTL (Through The Lens) meter to measure and control flash exposure. Non-TTL automatic flashes require that you manually set the aperture to match it's program. They usually have a scale on the flash that give usable flash to subject distances for a choice of aperture settings. To love this comment, log in above November 14, 2003

Jon Close		correction, the guide number equation should be f-number = (GN / D) x Square Root of (film ISO / 100) Example: GN = 22m or 72 feet. D = 18 feet film ISO = 400 f-number = (72/18) x SQRT(400/100) f-number = (4) x (2) f-number = 8 So you'd set the lens to f/8. To love this comment, log in above November 14, 2003

Esther Mishkowitz		i have the sb 26 flash on my nikon 90s however I would like to place my vivitar 283 behind the subject to separate the background from the subject. I have a subject wearing a black hat and suit against a navy velvet background! thanks for your help. To love this comment, log in above November 14, 2003

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