Exposures that seem impossibly brief can reveal a world we didn't know existed. In the 1930s, Dr. Harold Edgerton, a professor of electrical engineering at MIT, pioneered high-speed photography using a repeating electronic flash unit he patented called the stroboscope. As the inventor of the electronic flash, he popularized its use to freeze objects in motion, and you've probably seen his photographs of bullets piercing balloons and drops of milk forming a coronet-shaped splash.
Electronic flash freezes action by virtue of its extremely short duration—as brief as 1/50,000th second or less. Although the EOS 40D's built-in flash unit can give you these ultra-quick glimpses of moving subjects, an external flash, such as one of the Canon Speedlites, offers even more versatility. You can read more about using electronic flash to stop action in Chapter 7.
Of course, the 40D is fully capable of immobilizing all but the fastest movement using only its shutter speeds, which range all the way up to an astonishing 1/8,000th second. Indeed, you'll rarely have need for such a brief shutter speed in ordinary shooting. If you wanted to use an aperture of f/1.8 at ISO 100 outdoors in bright sunlight, for some reason, a shutter speed of 1/8,000th second would more than do the job. You'd need a faster shutter speed only if you moved the ISO setting to a higher sensitivity (but why would you do that?). Under less than full sunlight, 1/8,000th second is more than fast enough for any conditions you're likely to encounter.
Most sports action can be frozen at 1/2,000th second or slower, and for many sports a slower shutter speed is actually preferable—for example, to allow the wheels of a racing automobile or motorcycle, or the propeller on a classic aircraft to blur realistically. Figure 5.1 is another example. The 1/500 second shutter speed effectively stopped the soccer players in mid-stride, but allowed the fast-moving soccer ball to blur slightly. If the ball were perfectly sharp, it might look on first glance as if it had been glued to the player's leg. The blur tells us that this shot wasn't faked.
But if you want to do some exotic action-freezing photography without resorting to electronic flash, the 40D's top shutter speed is at your disposal. Here are some things to think about when exploring this type of high-speed photography:
■ You'll need a lot of light. High shutter speeds cut very fine slices of time and sharply reduce the amount of illumination that reaches your sensor. To use 1/8,000th second at an aperture of f/6.3, you'd need an ISO setting of 1,600—even in full daylight. To use an f/stop smaller than f/6.3 or an ISO setting lower than 1,600, you'd need more light than full daylight provides. (That's why electronic flash units work so well for high-speed photography when used as the sole illumination; they provide both the effect of a brief shutter speed and the high levels of illumination needed.)
■ Forget about reciprocity failure. If you're an old-time film shooter, you might recall that very brief shutter speeds (as well as very high light levels and very long exposures) produced an effect called reciprocity failure, in which given exposures ended up providing less than the calculated value because of the way film responded to very short, very intense, or very long exposures of light. Solid-state sensors don't suffer from this defect, so you don't need to make an adjustment when using high shutter speeds (or brief flash bursts).
■ No elongation effect. This is another old bugaboo that has largely been solved through modern technology, but I wanted to bring it to your attention anyway. In olden times, cameras used shutters that traveled horizontally. To achieve faster shutter speeds, focal plane shutters (located just in front of the plane of the sensor) open only a smaller-than-frame-sized slit so that, even though the shutter is already traveling at its highest rate of speed, the film/sensor is exposed for a briefer period of time as the slit moves across the surface.
At very short shutter speeds, and with subjects moving horizontally at very fast velocities, it was possible for the subject to partially "keep up" with the shutter if it were traveling in the same direction as the slit, producing an elongated effect. Conversely, subjects moving in the opposite direction of shutter motion could be compressed. Today, shutters like those in the 40D move vertically and at a higher maximum rate of speed. So, unless you're photographing a rocket blasting into space, and holding the camera horizontally to boot (or shooting a racing car in vertical orientation), it's almost impossible to produce unwanted elongation/compression.
■ Don't combine high shutter speeds with electronic flash. You might be tempted to use an electronic flash with a high shutter speed. Perhaps you want to stop some action in daylight with a brief shutter speed and use electronic flash only as supplemental illumination to fill in the shadows. Unfortunately, under most conditions you can't use flash in subdued illumination with your 40D at any shutter speed faster than 1/250th second. That's the fastest speed at which the camera's focal plane shutter is fully open: at shorter speeds, the "slit" described above comes into play, so that the flash will expose only the small portion of the sensor exposed by the slit during its duration. (Check out "High Speed Sync" in Chapter 7 if you want to see how you can use shutter speeds shorter than 1/250th second with certain Canon Speedlites, albeit at much-reduced effective power levels.)
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Although we usually tend to think of the digital camera as the best thing since sliced bread, there are both pros and cons with its use. Nothing is available on the market that does not have both a good and a bad side, but the key is to weigh the good against the bad in order to come up with the best of both worlds.