There’s an interesting quirk of working with strobes that allows a photographer to separate the flash exposure from the ambient exposure. It typically requires a bright ambient background and a shaded subject to be illuminated by the flash. Colloquially known as “dragging the shutter,” it’s a beautiful way to balance a strobe-lit subject with ambient backgrounds, and it can make for interesting portraits.
Dragging the shutter may be familiar to anyone who has used a point-and-shoot camera set to “night portrait” mode. It simply uses a slow shutter speed to add ambience to an image otherwise solely illuminated by flash. Without the slow shutter speed, the background would just go dark.
Done by the books, dragging the shutter produces a crisp, clear, well-illuminated background—whether that’s the glowing streets of Las Vegas at night or a brightly lit interior. It also produces a crisp, clear, well-lit subject, illuminated entirely by the flash. There’s nothing wrong with this approach to dragging the shutter, as it’s useful in all sorts of applications. But it misses an opportunity to introduce intentional motion blur to an otherwise static shot.
Something special happens when you intentionally break the rules that make for “good photographs” and drag the shutter to deliberately introduce motion blur. It can add energy to an otherwise plain picture, and it can help make a subject “pop” more dramatically from the background. It’s one of my favorite portrait lighting techniques, and a little goes a long way.
How It Works
Before we get into phase two of dragging the shutter—adding the motion—let’s first understand what gives the technique its teeth. It’s based on the simple principle that flash and ambient light can be controlled separately in the same exposure. That works entirely due to shutter speed.
Think about it: The flash fires much faster than 1/250th of a second. Its duration is so brief that the camera shutter speed, when set to the sync speed of 1/250th or slower, will always mean the shutter is open longer than the duration of the flash. (Flash power is actually a function of duration; a flash fires at a fixed intensity and simply changes duration as the power level is adjusted. So a full-power flash may fire for 1/1000th of a second, while at half power it’s only illuminated for 1/2000th. In each case, the flash turns on, cycles up to full power, and turns off much faster than the time during which the shutter is open.) All that means that adjusting the shutter speed makes no impact on the flash exposure, so long as you stay within the sync limit. To alter the flash, you’ve got to adjust the aperture or the ISO. Changing the shutter speed does nothing for the flash.
Where To Use It
You can employ dragging the shutter to introduce motion in the studio or out in the world, indoors or outside, in basically any situation where the background has some brightness to it and the subject is shaded (or can be shaded) so the ambient exposure doesn’t directly illuminate their face. The ideal situation could be low-level lighting in the background of an interior or even the glow of a post-sunset evening sky, but as long as there’s some illumination, it can work. I prefer for the background to be even brighter, as it allows the effect without having to use painfully long shutter speeds—and it makes it easier to balance with the flash. The aforementioned Vegas streetscape, for instance, is an ideal background for dragging the shutter, but so is a window, a white wall or any bright background. The brighter, the better, as far as I’m concerned.
Speaking of strobes, this technique does require one. But it can be a hot-shoe mounted flash or a fancy studio strobe; the key is simply that it isn’t a constant light, as the short duration of a flash is necessary to control the background ambience and the key light separately.
Indoors With A Bright Background
Working in any bright room or where window light fills the background, dragging the shutter is an ideal technique. By placing the subject far from the bright background and starting with an exposure that renders the background correctly exposed (or slightly under or over, as you see fit), you’ll also render the subject in silhouette—as long as you’re not inadvertently illuminating them, too. This can happen if they’re positioned near a light source (like another window or a ceiling light), or even if you have the model light on your strobe turned on. Because you’re making first an ambient exposure for the background, we want a strobe exposure alone for the subject, turning off such a model light—or flagging and moving to avoid extraneous light from hitting the subject—you can render them in pure shadow, ready to be illuminated by the strobe.
I start by choosing my ambient exposure, influenced by what I know I’m likely to get from my strobe—with a low ISO (say 100) and a moderate aperture (like ƒ/5.6) because I know my strobe won’t have any problem matching that level of intensity. I then choose a shutter speed in the neighborhood of 1/10 of a second to half a second, aiming for something in the 1/4 to 1/6 range—mostly because I find that with a lot of lenses, from 50mm primes to 150mm telephotos, the edge blur introduced by my natural handheld movements at these shutter speeds looks appropriate. After all, I just want a subtle blur at the edges of the subject—not something so blurry it obliterates all detail.
By dialing in the ambient and making a test shot to verify the background looks as I wish and the subject is silhouetted, I have an exposure that’s ready for the strobe. Now it’s just a function of powering up the strobe and adjusting its output and positioning until it looks appropriate. If it’s too bright and I can’t move it farther from the subject or adjust it to lower power, I’ll have to change the aperture—from, say, ƒ/8 to ƒ/11. Doing that will also impact the shutter speed, which will now need to move from, say, 1/8 to 1/4 in order to maintain the same ambient exposure. If it’s too dark, I’ll have to power it up to the max (though that’s not likely to happen with anything but the most underpowered speedlights), and then open up the aperture and shorten the shutter speed to compensate. As long as you manage to stay slower than 1/10, in my experience, you’ve got a good opportunity to introduce pleasing edge blur.
In The Studio
To create a bright background in the studio, I use a bright continuous light, such as a daylight-balanced HMI or even a blue gelled tungsten light (a 1k would be ideal) in order to create a bright background that will balance with the color temperature of my strobe. (Heck, if you want it to go orange, let it remain straight tungsten. And if you’d like it bluer, gel that HMI with a CTB.)
You can use lights of lower intensity for the background, but it tends to make it harder to balance well with the strobes. Most LEDs, for instance, just aren’t strong enough to make a bright background without resorting to painfully long shutter speeds or higher ISOs that then make it harder to bring down a strobe to equal them. The brighter the background ambience, the more options you have.
Speaking of options, another great option for lighting the background and foreground independently is to use flash delay to trigger two lights at two different times. Many radio transmitters, like the PocketWizard MultiMax, for instance, can be set to fire two strobes at different intervals. In practice, that means you can click the shutter and fire the first strobe as the key light immediately, then half a second later the second flash will fire before the shutter closes. Any movement between the first and second strobes will not create motion blur, per se, but rather an outline of the subject from the backlighting created by the second flash in conjunction with the natural movement of your handheld camera and subject. It doesn’t create quite the same effect as truly dragging the shutter with an ambient bright background, but it’s interesting and useful nonetheless.
Outdoors, Make Some Shade
If you’re going to try dragging the shutter outdoors, it helps to avoid trying it on a bright sunny day. Instead, choose a time where it’s easy to put the subject in shadow, while simultaneously having some illumination on the background. Sunrise and sunset, as well as evening hours with backgrounds illuminated by artificial light, work well and don’t require the photographer to go to the extra effort of creating a shaded subject. Should you find yourself faced with such a situation, you can look for natural shade or create it with large flags, such as “floppies” that can block bright natural light and allow you to create the primary subject exposure with the strobe. With the faster shutter speeds that come with bright sun, know that it’s going to be a little trickier to introduce motion blur.
Making Motion Blur With Light
Any time you’re dragging the shutter and handholding the camera, you’ve got the opportunity for motion blur—particularly evident at the edges of the subject as well as in the background details. Simply by handholding in conjunction with a slow shutter speed, movement will appear—and that’s a fine place to start. The longer the lens or the slower the shutter speed, the more blur you’re likely to see. The same goes for subjects with long hair, flowing garments or other elements that make movement easier to see.
But what if you want to be particular about what’s blurry and what’s not? Well, you can always put the camera on a tripod to ensure the background will be sharp and then any motion blur will come from the movement of the subject. This may naturally occur based on their pose or their action, or you may have to suggest movement to them: turning their head, shifting their weight or otherwise adding movement to the exposure to create the edge blur that adds a twist to a portrait. This will maintain a sharp background thanks to the tripod, but the subject’s motion will make for blurry edges to their outline, while the flash will do its job and freeze features that are both sharp and well exposed. This approach is all about experimentation to find exactly the right combination of aperture and movement to create the ideal look. And since you’re going “off book,” there’s no right way or wrong way for it to look.
If you want to exaggerate the motion blur or shape it in a particular way, you can do that, too. Rotating the lens’ zoom ring during exposure, for instance, makes for interesting background blurs while simultaneously setting off the subject from the background. Rotate the camera itself around the lens axis, and you can make radial, background blurs that appear to practically outline the subject. With a little forethought and an eye for experimentation, you can drag the shutter with deliberate motion and make some really interesting images—even if they break the rules for perfectly sharp pictures.