Dragging the Shutter: Mixing Strobe and Ambient Light
By Christopher Grey
Published by SekonicChristopher Grey goes in-depth about how to control color and motion in images taken with a combination of ambient light and strobes.
Photographer Chris Grey is an expert on lighting. He's written seven books on the subject, teaches classes and workshops, and is a strong advocate for precise control in the studio. To see more of his work, or to find his books and DVDs, visit his Web site.
Category: How-ToClick for more articles of: Christopher Grey
“Dragging the shutter” is an old-school term for keeping the shutter open long enough when using flash to register ambient light and make it part of the composition. While wedding photographers frequently allow 35 to 40% of ambient light to register along with the flash, it’s usually done at shutter speeds that rarely go below 1/60 second, to avoid background blur.
My studio is not set up for ambient light shoots, so I reserve this trick for those times when I’m on location and can find spots where the ambient light is both attractive and useful. Of course, the color of the ambient light is a concern. Incandescent light can be quite pretty while fluorescent light rarely makes the grade, at least for color images.
The first thing you should do at a location is to thoroughly scout it. Think in terms of mild to strong telephoto lenses, because normal and wide-angle lenses see too much and will pick up junky details that interfere with successful compositions. When you decide on a spot, set up the camera and roughly frame the shot. Meter the ambient light so you’ll know what you’re up against, and think of how bright you want it to be.
Next, set the strobe key light in position and attach the modifier of your choice. For the image below, I used a 4x6 softbox. Since I was constrained by space, it had to be placed relatively close to the shooting area. This meant I would be limited in how low I could power the unit—f/7.1 at ISO 400. I would have to gauge the effect of the ambient light based on that f-stop.
When we work with strobes, we rarely need to concern ourselves with shutter speed. Even at the maximum camera sync speed—1/250 second in many cases—the flash burst is over and done long before the shutter closes. However, when we mix sources we have to figure in the effect of the dimmest one, in this case a spotlight on the wall behind where my model would stand. Because the flash duration is so short, simply turning off the modeling light eliminated any problem with extra light from that unit.
I metered the scene to determine that an exposure of 1/3 second at f/7.1 would produce a fully toned image of the spotlight, complete with hotspot.
This was way more than I wanted. In my mind’s eye, I wanted only the hotspot to register, and quite softly at that. A couple quick tests told me that an exposure of 1/40 second would show only the hotspot, substantially dimmer than what I first saw and reduced to being only an accent light.
With the light in place, I made a test at 1/40 second and f/7.1 from my chosen angle. You can see a slight influence from the key light on the wall, but it’s not much since the key is so close to where the model will be and has lost a great deal of its strength by the time it gets there.
With my subject in place, the result is a great mix of strobe and ambient light. The spotlight becomes the only color in an almost neutral environment and adds much-needed warmth to the shot.
I wanted to use a corner in this location to lend a little dimension to the next shot. I set my 4x6 softbox in the adjacent room and aimed it at my new model’s back. I knew that she would be overexposed in places but felt that would actually add to the shot.
It was rather dim in the room, so I hung a white reflector panel on a stand to bounce some of the unused softbox light back onto the model. I was able to get the reflector high enough to see a passable nose shadow, but would have preferred another foot of height in the room to get a more graceful sweep under the cheek. The reflector did its job admirably, but I was not impressed with the shadow from the lock of hair on her right eye. She tried keeping it out of the way, but it had a mind of its own. This image was exposed at 1/200 second at f/7.1.
While the room was dim, there was enough ambient light from windows at camera-right to illuminate her at 1/30 second, f/7.1. The next set of images were almost magical. The slightly warmer light from the street, much softer than what had been reflected, gave her face a softness that appeared to wrap around from the softbox. It also did wonders for her naturally red hair and lit more of the wall she was standing against.
You can take this a step further (many steps, actually) by using your studio lights to create a mixed light setup and slowing the shutter dramatically.
Set your strobes for low power output, the lowest possible, and create a lighting scenario that’s appropriate for your moving subject. Use your light meter to measure the output of the strobe’s modeling lights to determine a shutter speed that will provide enough streaks from the highlights, perhaps a full second, maybe longer. It will depend on how fast the subject moves and how repetitious the action is. Measure the output of the strobe and tweak it until it matches the ambient light. This is only a starting point, and you should test it using your camera’s LCD to determine how much streaking is necessary to get the effect you want. When the two sources are properly matched, the image is a nice mix of motion and stopped-motion. This image was made at 1.3 seconds, f/16.
Yet another approach is to gel the strobe with a full CTO gelatin filter to convert its color temperature to that of incandescent light, approximately 3400K, while lighting the background with incandescent bulbs. The color temperature of the two lights will match, but the strobe will freeze action while the incandescent light will necessitate a longer exposure to register properly. Setting the camera’s white balance to incandescent will produce a neutral exposure.
The model was lit by a strobe that had been wrapped in a full CTO gel, to get its color temperature to that of incandescent light, and the modeling light was turned off. The background was lit by aiming an incandescent spotlight into my Reverse Cookie (see elsewhere in this book for more information) to splash a pattern of light back to the painted wall. The image of a belly dancer was made at 1/3 second, although the proper exposure was 1/6 second. To get the background brighter than “normal” I allowed the shutter to stay open for a full stop’s worth of extra light.
I watched my dancer carefully, timing my exposures to the peaks of the actions of her head. I didn’t care where her arms or torso went, only that her head stayed in approximately the same position for the length of the exposure. Needless to say, it took a number of frames to get a terrific image, with her head in place, the expression correct, and a beautiful semi-transparency to the parts of her that moved throughout the exposure.