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Chapter 5. Using the Flash

Chapter 5. Using the Flash

Your 7D has a flip-up electronic flash unit built in, but you can also use an external flash (or “strobe” or “Speedlite”), either mounted on the 7D’s accessory shoe or used off-camera and linked with a cable or triggered by a slave light (which sets off a flash when it senses the firing of another unit). Consider using electronic flash:

  • When you need extra light. Flash provides extra illumination in dark environments where existing light isn’t enough for a good exposure, or is too uneven to allow a good exposure even with the camera mounted on a tripod.

  • When you don’t need a precise preview of lighting effects. Although some external units have a modeling flash feature that gives a preview of the strobe’s effects, the “modeling” flash may not give you a precise look at what you’re going to get. Unless you’re using a studio flash with a full-time modeling lamp, electronic flash works best when you are able to visualize its effects in your mind, or don’t need a precise preview.

  • When you need to stop action. The brief duration of electronic flash serves as a very high “shutter speed” when the flash is the main or only source of illumination for the photo. Your 7D’s shutter speed may be set for 1/250th second during a flash exposure, but if the flash illumination predominates, the effective exposure time will be the 1/1,000th to 1/50,000th second or less duration of the flash, because the flash unit reduces the amount of light released by cutting short the duration of the flash. However, if the ambient light is strong enough, it may produce a secondary, “ghost” exposure, as I’ll explain later in this chapter.

  • When you need flexibility. Electronic flash’s action-freezing power allows you to work without a tripod in the studio (and elsewhere), adding flexibility and speed when choosing angles and positions. Flash units can be easily filtered, and, because the filtration is placed over the light source rather than the lens, you don’t need to use high-quality filter material.

  • When you can use—or counter—flash’s relatively shallow “depth-of-light” (the inverse square law). Electronic flash units don’t have the sun’s advantage of being located 93 million miles from the subject, and suffer from the effects of their proximity. The inverse square law dictates that as a light source’s distance increases from the subject, the amount of light reaching the subject falls off proportionately to the square of the distance. In plain English, that means that a flash or lamp that’s eight feet away from a subject provides only one-quarter as much illumination as a source that’s four feet away (rather than half as much). (See Figure 5.1.) You can use this effect to separate subjects located at different distances thanks to the differing amount of illumination each receives. But when you want a larger area blanketed more evenly with illumination, you have to counter the effects of the inverse square law with supplemental lighting, slow shutter speeds (which allow ambient light to register along with the flash), or by repositioning your subjects so all are within your flash’s depth-of-light coverage.

    Figure 5.1. A light source that is twice as far away provides only one-quarter as much illumination.


  

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