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Saturday, September 1, 2007

Seeing in Slo-Mo

How to get motion blurs that will add a new dimension to your photography

The Flash Blur: Combining Flash And Ambient Light

With nearby moving subjects, you can combine flash with a slow shutter speed. The brief flash duration will sharply freeze an image of the nearby subject, while the slow shutter speed produces a blurred ghost image from the ambient light. On a windy day, you can use flash to "freeze" nearby foreground flowers while background blooms, beyond flash range, blur.

You also can combine flash with ambient light to capture both detail and a sense of frenzy in images of birds battling over territory or female birds (frequently seen in breeding season). The long exposure time will record motion blur, while the brief flash exposure will freeze a sharp image of the combatants.

Most TTL flash systems automatically balance the flash and background exposures, but many D-SLRs provide both ambient light and flash exposure compensation, so you can adjust the ratio to your liking. Using a stop or two of minus exposure compensation for the ambient light will darken the background, a dramatic albeit not natural effect.

If your subject is moving across the frame, try rear sync flash, available with most of today’s SLRs and higher-end flash units. Normal front sync will produce ghost-image speed streaks in front of the moving subject, while rear sync will produce speed streaks behind the moving subject, a more natural effect.

The Digital Advantage

Digital cameras offer a few huge advantages for experimenting with blur effects: You can check the effect on the camera's LCD monitor right after shooting the image, and the metadata records the exposure time and other data for each shot, saving you the trouble (and shooting delays) of keeping notes manually. When you review your images on your computer monitor, just check the metadata to see what exposure times produced the results you like best.

Reciprocity Failure

If you shoot your blur effects on film, you may encounter reciprocity failure, a loss of film speed that occurs at very long (and very short) exposure times and causes underexposure unless you give more exposure than a meter reading calls for. Because color films have three or four emulsion layers, each of which loses speed at a different rate, color shifts occur along with overall underexposure.

Each film has its own reciprocity characteristics. Reciprocity data for a given film (telling you how much exposure compensation is needed for various exposure times and what corrective filtration is required with color films) is sometimes packaged with the film; otherwise, you can request it from the film manufacturer or find it on the company’s Website. With most of today’s films, reciprocity failure doesn’t require correction until exposure times exceed 10 seconds (60 seconds with some films).

Digital SLRs are minimally affected (if at all) by the exposure and color effects of long-exposure reciprocity failure, but image noise may increase as exposure times grow longer. If you’re shooting long-exposure blur effects with a digital camera, you should activate its long-exposure noise-reduction feature. If you find image noise to be a problem in long-exposure images, you might want to invest in a good noise-reduction software product such as ASF’s Digital GEM (www.asf.com), Imagenomic’s Noiseware (www.imagenomic.com) or PictureCode’s Noise Ninja (www.picturecode.com).

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