Home How-To Tip Of The Week Mixing Flash And Daylight—09/05/11
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Monday, September 5, 2011

Mixing Flash And Daylight—09/05/11

How to control strobe and ambient independently in the same exposure

This Article Features Photo Zoom
It was a grand moment in my life when the light bulb finally flipped on in my head, and I grasped the concept of controlling strobe and ambient exposures separately in a single exposure. It stems from the fact that a flash duration is so short—1/2000th of a second, say—that adjusting the shutter speed has no effect on the flash exposure. To control the flash exposure, you have to adjust the aperture. Therefore, adjustments to shutter speed only impact the ambient exposure, so you can control daylight and flash exposures separately in a single push of the button.

In practice, that means you can over- and underexpose daylight for richer color and darker skies, and then use your flash as more than fill, but as a main light of varying intensities, too. For instance, let's say the basic daylight exposure is 1/100th at ƒ/16. If you were photographing a pot full of flowers against a sunny blue sky—as I did—you've got lots of options when it comes to balancing the strobe with the ambient.

No Flash Full Flash Half Flash Darker Ambient,
Half Flash
Darker Ambient,
Full Flash

First is the ambient-only exposure. At the “normal” daylight exposure with no flash fill, the sky and background look nice, but the flowers are dark and backlit and in need of some fill.

The second exposure shows the normal daylight exposure with the flash at full power. It also looks fine, but the balance between daylight background and foreground flash fill isn't quite right. The flash is definitely too strong in this composition. In this case, the easiest way to compensate for that is to cut the flash power in half and keep the exposure the same. When I do that, the ratio looks better, but still not quite right.

What I want is a deeper, darker blue sky for richness and color that seems more saturated. To do that, I want to underexpose the ambient by a full stop without changing the flash exposure. And to do that, you need to go back to that guiding principle: the only way to adjust flash exposure in-camera is to change the aperture, not the shutter speed.

So instead of 1/100th of a second I'll change it to 1/200th. Sure enough, the flash exposure looks the same, but now the background is deeper blue. More dramatic. But still, something isn't quite right.

With this new darker background it turns out that the original full-power flash exposure would probably work better. So back on full power and voila, the flowers are brightly exposed and the background is a bit underexposed for maximum drama. This is a great way to make your pictures pop when mixing daylight ambience with a strobe.

And just to help drive the point home, let's consider the other ways we could mix those exposures without adjusting flash power. Since 1/100th at ƒ/16 made the flash too bright, 1/100th at ƒ/22 would make the flash darker, but it would also make the ambient darker too. In order to compensate, a shutter speed of 1/50th would make the ambient look normal with the ƒ/22 aperture that brings the flash exposure down one stop.

But then we decided that the ambient was too bright, so we underexposed it by a stop—back to 1/100th at ƒ/22. And at that point the strobe wasn't bright enough to make the image pop, so finally we'd want to end up at ƒ/16 for the strobe exposure, and to keep the ambient a full stop under the shutter speed would go to 1/200th.

It's a technique that may be tough to wrap your head around, but keep trying and you're sure to get it and eventually have your own Eureka moment too. After that, it's a simple technique that you'll put into practice any time you're working with flash and ambience.


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