BetterPhoto Q&A
Category: New Answers

Flash problems

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October 17, 2005

Simply said, there are just some limitations to what a built-in flash can do. And flash lighting can become a little complificated because there are so many variables. So it might be worthwhile to understand what you're trying to do.

First of all, understand that film (and digital sensors) have a limited range of sensitivity, or latitude. That is, the brightest points in a properly exposed image can only be about 7-8 "stops" as bright as the darkest sections of that image, otherwise details will be lost to the light side ("blown out") or in the shadow areas. The human eye is more sensitive than this - so what looks to your naked eye as a reasonable dark-to-light ratio may well be beyond the camera's or film's capability to record. It is for this reason that you see photographers using a flash on the beach at 1PM - the idea is to bring the shadowed faces up to the level of brightness of the surf and sand in the background. Does that make sense?

So job #1 is to bring everything within those 7-ish stops worth of light variance. Now, to do that might require a rather powerful light source if you're having to deal with a distant subject (like someone on stage) or, more often, with a subject in the foreground but a distant and dark background area.

You also need to understand how automatic flashes work, in general. Basically, the bulb in an electronic (non-automatic) flash fires for 1/1000th of a second. With an automatic flash, there is either a sensor in the flash unit, or, on more modern cameras (like yours) a meter in the camera that actually reads the light as it returns from the subject and kills the flash tube prior to that 1/1000th of a second time. So an auto flash might "quench" in 1/30,000th of a second if there is enough ambient lighting to give an overall proper exposure. I know it sounds incredibly quick, but no kidding - that's what's going on in there. Of course, to slow wetware based humanoids like us, no matter what it all seems pretty instantaneous, but the equipment has that level of control.

So, say you're taking a shot of your friend outdoors at night, where the shrubbery is 20-30 feet behind the subject. Your camera's automatic exposure system does not "know" the situation - it is only trying to get the average light over the entire image to some overall amount. So, when you click the shutter and the flash fires, if your friend's face takes up most of the shot the flash might quench very quickly and you'll get a nicely exposed face with a very very dark background. On the other hand, if your friend steps back a bit (or you're using a wider angle lens) then his face is only taking up, say, 5% of the total scene. If his clothes are dark, that plus the dark distant background is going to cause the flash unit to push out its maximum power. Thus, his face becomes a ghost even though there still wasn't enough light output from your little on-camera flash to illuminate the background anyway. Thus, your dilemma.

So, the options you have to fix any of this are to get a bigger flash (though there are limits to that, of course) or, knowing in advance that your meter will be misled, set your camera to underexpose by a stop or two. This latter step will cause the flash to figure "well, okay, she wants me to cut out way before I normally would" but of course this is exactly what you would like to happen.

I hope that made some sense - flash can be a bit overwhelming once you get into it. And know that when you see those magnificent shots in Sports Illustrated the shooters actually rig ginormous flashes in the rafters above the b-ball court or boxing ring which are essentially invisible to the athletes and fans but which are enough to properly illuminate the court for the great cover shot.

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October 17, 2005

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October 18, 2005