Now that you have a good understanding of exposure under your belt, you'll want to master some of the other techniques that can contribute to great images. In this chapter, I'm going to show you how to work with some additional exposure options, use the automatic and manual focusing controls available with the Canon EOS Digital Rebel XTi, and explain some of the many ways you can fine-tune your images with optimized white balance, sharpening, tonal values, and color.
In Chapter 4, you learned techniques for getting the right exposure, but I haven't explained all your exposure options just yet. You'll want to know about the kind of exposure settings that are available to you with the Canon Digital Rebel XTi. There are options that let you control when the exposure is made, or even how to make an exposure that's out of the ordinary in terms of length (time or bulb exposures). The sections that follow explain your camera's special exposure features, and even discuss a few it does not have (and why it doesn't).
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 Rebel XTi'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 freeze action in Chapter 7.
Of course, the XTi is fully capable of stopping all but the fastest movement using only its shutter speeds, which range all the way up to an astonishing 1/4000th 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 1/4000th 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 (for some unknown reason). Under less than full sunlight, 1/4000th second is more than fast enough for any conditions you're likely to encounter.
Most sports action can be frozen at 1/2000th 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 1/2000th second shutter speed effectively stopped the batter in mid-stroke, but allowed the 90-mph fastball to blur. If the fastball were perfectly sharp, it might look as if it had been glued to the bat. 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 XTi'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 1/4000th second at an aperture of f/8 you'd need an ISO setting of 1600— even in full daylight. To use an f/stop smaller than f/8 or an ISO setting lower than 1600, you'd need more light than full daylight provides. (That's why electronic flash units work so well for high-speed photography under the right conditions; they provide both the 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. The consensus today is that 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 (nor for very long exposures, either).
■ 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 XTi 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 with your XTi at any shutter speed faster than 1/200th 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 faster than 1/200th second with certain Canon Speedlites, albeit at much-reduced effective power levels.)
You can have a lot of fun exploring the kinds of pictures you can take using very brief exposure times, whether you decide to take advantage of the action-stopping capabilities of your electronic flash, or work with the Canon Digital Rebel XTi's faster shutter speeds. Here are a few ideas to get you started:
■ Revealing images. Fast shutter speeds can help you reveal the real subject behind the façade, by freezing constant motion to capture an enlightening moment in time. Legendary fashion/portrait photographer Philippe Halsman used leaping photos of famous people, such as the Duke and Duchess of Windsor, Richard Nixon, and Salvador Dali to illuminate their real selves. Halsman said, " When you ask a person to jump, his attention is mostly directed toward the act ofjumping and the mask falls so that the real person appears." Try some high-speed portraits of people you know in motion to see how they appear when concentrating on something other than the portrait.
■ Create unreal images. High-speed photography can also produce photographs that show your subjects in ways that are quite unreal. A helicopter in mid-air with its rotors frozen, or a motorcyclist banking into a turn, but with all motion stopped so that the rider and machine look as if they were standing still at an odd angle, make for an unusual picture. When we're accustomed to seeing subjects in motion, seeing them stopped in time can verge on the surreal.
■ Capture unseen perspectives. Some things are never seen in real life, except when viewed in a stop-action photograph. Edgerton's balloon bursts were only a starting point. Freeze a hummingbird in flight for a view of wings that never seem to stop. Or, capture the splashes as liquid falls into a bowl, as shown in Figure 5.2. No electronic flash was required for this image (and wouldn't have illuminated the water in the bowl as evenly). Instead, a clutch of high intensity lamps and an ISO setting of 1600 allowed the Rebel XTi to capture this image at 1/2000th second.
■ Vanquish camera shake and gain new angles. Here's an idea that's so obvious it isn't always explored to its fullest extent. A high enough shutter speed can free you from the tyranny of a tripod, making it easier to capture new angles, or to shoot quickly while moving around, especially with longer lenses. I tend to use a monopod or tripod for almost everything when I'm not using an image-stabilized lens and end up missing some shots because of a reluctance to adjust my camera support to get a higher, lower, or different angle. If you have enough light and can use an f/stop wide enough to permit a high shutter speed, you'll find a new freedom to choose your shots. I have a favored 170mm-500mm lens that I use for sports and wildlife photography, almost invariably with a tripod, as I don't find the "reciprocal of the focal length" rule particularly helpful in most cases. (I would not handhold this hefty lens at its 500mm setting with a 1/500th second shutter speed under most circumstances.) However, at 1/2000th second or faster, it's entirely possible for a steady hand to use this lens without a tripod or monopod's extra support, and I've found that my whole approach to shooting animals and other elusive subjects changes in high-speed mode. Selective focus allows dramatically isolating my prey wide open at f/6.3, too.
Longer exposures are an additional doorway into another world, showing us how even familiar scenes can look much different when photographed over periods measured in seconds. At night, long exposures produce streaks of light from moving, illuminated subjects like automobiles or amusement park rides. Concerts take on a new look when performers are photographed using exposures of a second or two, as you can see in Figure 5.3. Extra-long exposures of seemingly pitch-dark subjects can reveal interesting views using light levels barely enough to see by. At any time of day, including daytime (in which case you'll often need the help of neutral density filters to make the long exposure practical), a slow shutter speed can cause moving objects to vanish entirely, because they don't remain stationary long enough to register in a photograph.
There are actually three common types of lengthy exposures: timed exposures, bulb exposures, and time exposures. The Rebel XTi offers only the first two, but once you understand all three, you'll see why Canon made the choices it did.
■ Timed exposures. These are long exposures from 1 second to 30 seconds, measured by the camera itself. To take a picture in this range, simply use Manual or Tv modes and use the Main Dial to set the shutter speed to the length of time you want, choosing from preset speeds of 1.0, 1.5, 2.0, 3.0, 4.0, 6.0, 8.0, 10.0, 15.0, 20.0, or 30.0 seconds (if you've specified 1/2 stop increments for exposure adjustments), or 1.0, 1.3, 1.6, 2.0, 2.5, 3.2, 4.0, 5.0, 6.0, 8.0, 10.0, 13.0, 15.0, 20.0, 25.0, and 30.0 seconds (if you're using 1/3 stop increments). The advantage of timed exposures is that the camera does all the calculating for you. There's no need for a stop-watch. If you review your image on the LCD and decide to try again with the exposure doubled or halved, you can dial in the correct exposure with precision. The disadvantage of timed exposures is that you can't take a photo for longer than 30 seconds.
■ Bulb exposures. This type of exposure is so-called because in the olden days the photographer squeezed and held an air bulb attached to a tube that provided the force necessary to keep the shutter open. Traditionally, a bulb exposure is one that lasts as long as the shutter-release button is pressed; when you release the button, the exposure ends. To make a bulb exposure with the XTi, set the camera on Manual mode and use the Main Dial to select the shutter speed immediately after 30 seconds—buLB. Then, press the shutter to start the exposure, and release it again to close the shutter. If you'd like to simulate a time exposure (described above), you can use the Canon RS-60E3 that attaches to the terminal on the left side of the camera under the rubber cover. You can also use the infrared wireless controllers RC-1 and RC-5.
■ Time exposures. This is a setting found on some cameras to produce longer exposures. With cameras that implement this option, the shutter opens when you press the shutter-release button, and remains open until you press the button again. Usually, you'll be able to close the shutter using a mechanical cable release or, more commonly, an electronic release cable. The advantage of this approach is that you can take an exposure of virtually any duration without the need for special equipment (the tethered release is optional). You can press the shutter-release button, go off for a few minutes, and come back to close the shutter (assuming your camera is still there). The disadvantages of this mode are exposures must be timed manually, and with shorter exposures it's possible for the vibration of manually opening and closing the shutter to register in the photo. For longer exposures, the period of vibration is relatively brief and not usually a problem—and there is always the release cable option to eliminate photographer-caused camera shake entirely. While the XTi does not have a built-in time exposure capability, you can simulate it with the bulb exposure technique, described previously.
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