How to Freeze Motion With Flash (High Speed Photography)

Seeing something that moves very fast frozen in a brief moment of time will always be fascinating. It's like peering into a new world, with new, interesting elements at play. But how to do this well has frustrated beginners and advanced shooters alike. One method to freeze ultra-fast motion is with the use of manual flash. It requires some setup, but when you understand the principles at play, you can make some amazing images in your own living room with a beginner photo kit. Overview Before getting into all the technical details, it will help to have an overview of what we're trying to achieve. We want a fast-moving subject, like a piece of fruit splashing into a glass of water, to be frozen in time. Achieving the shutter speed necessary to do this is a challenge, especially in low light situations. The solution is to use manual flash. The burst of light from a flash, such as a Speedlight, is ultra-short, between 1/10,000 second and 1/40,000 second for most Speedlights -- upward of 1/100,000 second for studio strobes. When flash is contributing 100 percent of the light, its duration becomes the effective shutter speed. The key is getting flash to be the only light in the exposure.

What You'll Need

1. Two external Speedlights, or one Speedlight and an SLR capable of manual shooting with built-in flash commander mode.

2. Snoot (a flash modifier that funnels light into a beam). If you don't have a snoot, in a pinch you could wrap some Cinefoil around the flash head so that it funnels the light through a narrow path, maybe slightly wider than the flash head, and just use a rubber band to hold it on.

3. Light stand or support recommended.

4. Something with fast motion to freeze; a common subject is a glass of water with something to drop into it.

Explanation Ambient light is much lower intensity than flash, so it needs time to burn into the exposure. Indoor ambient light is especially low and usually requires high ISOs, low shutter speeds and wide apertures to record. In other words, you set up the camera to let a lot of light in. So what would happen if you went the opposite direction and lowered the ISO, sped up the shutter, and cranked down the aperture so it let in much less light? No ambient light would record, and for this experiment that's exactly what you want. You want a black frame to start with because you're going to paint in with flash. Even when you block ambient light from the exposure by closing down the aperture and lowering the ISO, flash is intense enough to record, so it will be the only light in the exposure, which also means its duration becomes the effective shutter speed. Flash is much more intense than ambient light and therefore needs to burn at a much shorter duration on the sensor. You can see just with your eyes how blinding a flash can be. The high intensity burst of flash is such a short duration that it gets burned onto the sensor and turned off in just a fraction of the time it takes for the shutter to open and close. This means it doesn't matter what the actual shutter speed is; the flash will sneak into the exposure regardless. To control exposure for the ambient light and flash separately, use the camera in manual mode. Not only does this work for all models, it also makes it easier to control the exposure and show you the effects of exposure control on each light source. You probably already know aperture controls the opening in the lens and the amount of light coming through it. That means aperture has the power to let in more or less of all light coming through the lens, be it ambient or flash; it's not time dependent. Closing down the aperture decreases the total light reaching the sensor, while opening it will increase total light. This means the aperture controls the amount of flash getting through the lens. Since the shutter speed doesn't matter for the exposure, you can set it to anything that blocks ambient light. However, the firing of the flash must sync with the shutter, or the closing of the shutter may block some of the flash from the exposure, creating a black bar in the image. To prevent this, set the shutter speed to your camera's sync speed, 1/200 to be safe.

Walkthrough Set up your stage indoors in average light or even with the lights off. Pick a dark or at least plain background away from light sources. If you're doing off-camera flash, place your light about 45 degrees from the camera (4 o'clock or 8 o'clock position to subject's front), making sure the wireless eye, usually indicated by some sort of symbol, has line of sight to the camera. If you have a snoot, place it on the flash and point the flash directly at the subject. Set the ISO to the lowest number or least sensitivity. Use a light meter or the camera's meter to get a reading of the ambient light. Set the shutter speed to something no faster than the camera's sync speed. Then, use a high f-number of at least f/10 to block the rest of the ambient light. The camera should be telling you you're way underexposed, which is good. You'll be painting the subject with flash, not ambient light. Turn all flashes off and take a test shot. With digital, you can tell right away if the image is black. If there's any detail at all, close down the aperture more and test until you get a completely black image. If you can, increase the shutter speed a bit also. Now turn the flash on. If you're using flash on camera, switch the flash to manual mode and go down to 1/32 power. If you're doing off-camera flash, set the remote flash to remote mode and place another Speedlight or the built-in Speedlight in commander mode, and then adjust power on the remote flash using the commander. If necessary, review the procedure in your camera and flash manuals for using a remote Speedlight with a commander. Take a test shot without the subject in action. The flash should fire. You can judge the exposure using just the LCD if you want, but a more accurate method is to use a hand-held light meter in flash mode. Simply adjust the exposure by moving the flash power up or down. If you go all the way down and the exposure is still too bright, move the flash further away or close the aperture more. Compose your image and get the focus correct. Count down and have your assistant drop a strawberry, for example, into the glass of water, and shoot just as it splashes down. It will take several shots to get the timing and splash just right, but all the drops of water will be frozen. Review the exposure and adjust as necessary. The adjustment is unnecessary if you've metered the flash and set the camera to the meter's reading. Shoot until you get exactly what you want.

Using a snoot is essential because it helps direct the light toward the subject so that it really pops out, and it prevents light from spilling onto the background and other parts of the scene, which would create distractions in the final image. Using a light stand also helps in positioning the flash off camera to the side, so light falls off out of frame instead of onto the background.

Summary With an understanding of flash and basic equipment, you can produce stunning images almost anywhere. Experiment with different flash settings and positions, multiple lights, using the built-in light for added front lighting, or even colored backgrounds. You can use full auto TTL flash, but I find that it often overexposes the subject and is harder to control precisely. With manual mode, you know that each increment is a full stop of brightness. This experiment not only gives you some cool images but it will help you learn some of the fundamentals of flash. I think you'll be excited by the results. If you have any questions, please email me.