Nikon D7000 manual How Electronic Flash Works

408 David Busch’s Nikon D7000 Guide to Digital SLR Photography

easily see that the built-in flash would begin to peter out at about 20 feet, where you’d need an aperture of f/2 (with a fast prime lens) at ISO 100. Of course, in the real world you’d probably bump the sensitivity up to a setting of ISO 800 so you could use a more practical f/5.6 at that distance.

Today, guide numbers are most useful for comparing the power of various flash units, rather than actually calculating what exposure to use. You don’t need to be a math genius to see that an electronic flash with a GN in feet of, say, 157 (like the SB-900) would be a lot more powerful than your built-in flash. At ISO 100, you could use f/8 instead of f/2 at 20 feet, an improvement of about 4 stops.

How Electronic Flash Works

The bursts of light we call electronic flash are produced by a flash of photons generated by an electrical charge that is accumulated in a component called a capacitor and then directed through a glass tube containing xenon gas, which absorbs the energy and emits the brief flash. For the pop-up flash built into the D7000, the full burst of light lasts about 1/1,000th of a second and provides enough illumination to shoot a subject 10 feet away at f/4 using the ISO 100 setting. In a more typical situation, as noted earlier, you’d use ISO 200, f/5.6 to f/8 and photograph something 8 to 10 feet away. As you can see, the built-in flash is somewhat limited in range; you’ll see why external flash units are often a good idea later in this chapter.

An electronic flash (whether built in, attached to the accessory shoe, or connected to the D7000 through a cable plugged into a hot shoe adapter in the accessory shoe) is triggered at the instant of exposure, during a period when the sensor is fully exposed by the shutter. As I mentioned earlier in this book, the D7000 has a vertically traveling shutter that consists of two curtains. The first curtain opens and moves to the opposite side of the frame, at which point the shutter is completely open. The flash can be trig- gered at this point (so-called first-curtain sync), making the flash exposure. Then, after a delay that can vary from 30 seconds to 1/250th second (with the D7000; other cam- eras may sync at a faster or slower speed), a second curtain begins moving across the sensor plane, covering up the sensor again. If the flash is triggered just before the sec- ond curtain starts to close, then second-curtain sync is used. In both cases, though, a shut- ter speed of 1/250th second is the maximum that can be used to take a photo, unless you’re using the high-speed 1/320th second sync.

Figure 12.7 illustrates how this works, with a fanciful illustration of a generic shutter (your D7000’s shutter does not look like this, and some vertically traveling shutters move bottom to top rather than the top-to-bottom motion shown). Both curtains are tightly closed at upper left. At upper right, the first curtain begins to move downwards, start- ing to expose a narrow slit that reveals the sensor behind the shutter. At lower left, the first curtain moves downwards farther until, as you can see at lower right in the figure, the sensor is fully exposed.