 Gerald Coppola |
Flash Power Are the guide numbers on an on camera flash unit used the same way for indoor as well as outdoor portraiture? In other words, in a situation where there is bright sunlight, and I want to use fill flash, is the formula GN/D = fStop still applicable? If not, how would I know the flash will be powerful enough to open up the shadows?
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Mark Feldstein |
Well, yes and no. The guide number on a flash is essentially a measurement of it's power output converted to more practical terms of distance to subject and f-stop settings. Whether you're working indoors or out, the output of your flash will be the same unless, of course, it has some adjustments that allow you to turn down the power output, say by 50% or in 1/4 f-stop increments. Soooooooooooo, given whatever shooting situation you're in, including the brightness of ambient light you're working in (whether it's indoors or out), you should set your flash for full power or ratio it down for fill flash. For example, when you're working outdoors maybe doing portraits of people, all you may need to do is set your camera for available light and then use your flash to add just a small 1/4 power burst of flash to fill in the shadows around the eye sockets of your subjects. Your guide number will tell you what f-stop to use to use full power. For fill light in the situation you mentioned, divide it by four [25%] or even by 2 [50%] and experiment a bit to decide which gives you the most pleasing results. Bracket your exposures until you get the hang of it. ;>) Take it light.
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Gerald Coppola |
Thanks, Mark. So, assuming a bright sunlite on the subject...I would still use the f-stop the formula gives me, bracket the exposure, and take the power of the flash down incrementally to find ther most pleasing fill effect, correct?
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Mark Feldstein |
Yep. that's pretty much it. Although when you're outdoors in bright sunlight, especially during the hours of say 10-2 when the sun is more overhead, look at the eyes of your subjects. If they appear to be deep in shadow, setting your flash to fill in at about 1/2 - 3/4 of an f-stop will probably be a pretty good starting point for your bracketing. And when you do bracket Ger, all you need to do is go about 1/2 a stop wider and 1/2 a stop down. I'm kind of assuming you're working with a digital camera and not film. If you are working with film, for transparency stock you should be working at 1/2 stop underexposed initially anyway to get better color saturation. For color negative films you can easily go 1/2 to 1 full f-stop OVERexposed and that will produce better saturation for that type of film. With b&w, you essentially need to expose for the shadows and process the film for the highlights. That's a metering exercise. :>)) Anything else Gerald, just hollar. We'll be glad to help you out.
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