Fixing Exposures with Histograms
While you can often recover poorly exposed photos in your image editor, your best bet is to arrive at the correct exposure in the camera, minimizing the tweaks that you have to make in post-processing. However, you can't always judge exposure just by viewing the image on your XTi's LCD after the shot is made. Ambient light may make the LCD difficult to see, and the brightness level you've set can affect the appearance of the playback image.
Instead, you can use a histogram, which is a chart displayed on the Digital Rebel's LCD that shows the number of tones being captured at each brightness level. You can use the information to provide correction for the next shot you take. The XTi offers two histogram variations: one that shows overall brightness levels for an image, and an alternate version that separates the red, green, and blue channels of your image into separate histograms.
Both types are charts that include up to 256 vertical lines on a horizontal axis that show the number of pixels in the image at each brightness level, from 0 (black) on the left side to 255 (white) on the right. The more pixels at a given level, the taller the bar at that position. If no bar appears at a particular position on the scale from left to right, there are no pixels at that particular brightness level.
A typical histogram produces a mountain-like shape, with most of the pixels bunched in the middle tones, with fewer pixels at the dark and light ends of the scale. Ideally, though, there will be at least some pixels at either extreme, so that your image has both a true black and a true white representing some details. Learn to spot histograms that represent over- and underexposure, and add or subtract exposure using an EV modification to compensate.
For example, Figure 4.15 shows the histogram for an image that is badly underexposed. You can guess from the shape of the histogram that many of the dark tones to the left of the graph have been clipped off. There's plenty of room on the right side for additional pixels to reside without having them become overexposed.
To view histograms on your screen, press the Disp. button while an image is shown on the LCD. Keep pressing the button until the histogram(s) are shown. The display will cycle between basic information, advanced information (with histogram display), and no information (a screen with only the image shown). You'll also see a thumbnail at the left side of the screen with your image displayed. Overexposed areas will blink, as shown by the dark (but non-blinking on the printed page) area in Figure 4.14, which is your prompt to reduce the exposure using exposure compensation. (See the "Making EV Changes" sidebar earlier in this chapter.) To change your histogram type from Brightness to RGB, use the Histogram setting in the Playback menu, as discussed in Chapter 3.
Figure 4.14 A histogram shows the relationship of tones in an image.
Figure 4.14 A histogram shows the relationship of tones in an image.
Or, a histogram might look like Figure 4.16, which is overexposed. In either case, you can increase or decrease the exposure (either by changing the f/stop or shutter speed in manual mode or by adding or subtracting an EV value in autoexposure mode) to produce the corrected histogram shown in Figure 4.17, in which the tones "hug" the right side of the histogram to produce as many highlight details as possible. See "Making EV Changes" earlier in this chapter, for information on dialing in exposure compensation.
The histogram can also be used to aid in fixing the contrast of an image, although gauging incorrect contrast is more difficult. For example, if the histogram shows all the tones bunched up in one place in the image, the photo will be low in contrast. If the tones are spread out more or less evenly, the image is probably high in contrast. In either case, your best bet may be to switch to RAW (if you're not already using that format) so you can adjust contrast in post processing. However, you can also change to a user-defined Picture Style (User Def. 1, User Def. 2, or User Def. 3) in the Picture Style menu, with contrast set lower (-2 to 0) or higher (0 to +2) as required. You'll find more about using Picture Styles in Chapter 5.
Your Canon Digital Rebel XTi includes seven Basic Zone shooting modes that can automatically make all the basic settings needed for certain types of shooting situations, such as portraits, landscapes, close-ups, sports, night portraits, and "no-flash zone" pictures. They are especially useful when you suddenly encounter a picture-taking opportunity and don't have time to decide exactly which Creative Zone mode you want to use. Instead, you can spin the Mode Dial to the appropriate Basic Zone mode and fire away, knowing that, at least you have a fighting chance of getting a good or usable photo.
Basic Zone modes are also helpful when you're just learning to use your XTi. After you've learned how to operate your camera, you'll probably prefer one of the Creative Zone modes that provide more control over shooting options. The Basic Zone scene modes may give you few options or none at all. Here are the modes available:
■ Full Auto. This is the mode to use when you hand your camera to a total stranger and ask him or her to take your picture posing in front of the Leaning Tower of Pisa. All the photographer has to do is press the shutter-release button. Every other decision is made by the camera's electronics.
■ Portrait. This mode tends to use wider f/stops and faster shutter speeds, providing blurred backgrounds and images with no camera shake. If you hold down the shutter release, the XTi will take a continuous sequence of photos, which can be useful in capturing fleeting expressions in portrait situations.
■ Landscape. The XTi tries to use smaller f/stops for more depth-of-field, and boosts saturation slightly for richer colors.
■ Close-Up. This mode is similar to the Portrait setting, with wider f/stops to isolate your close-up subjects and high shutter speeds to eliminate the camera shake that's accentuated at close focusing distances. However, if you have your camera mounted on a tripod or you are using an image-stabilized (IS) lens, you might want to use the Creative Zone aperture priority (Av) mode instead, so you can specify a smaller f/stop with additional depth-of-field.
■ Sports. In this mode, the XTi tries to use high shutter speeds to freeze action, switches to High Speed Continuous drive to allow taking a quick sequence of pictures with one press of the shutter release, and uses AI Servo AF to continually refocus as your subject moves around in the frame. You can find more information on autofocus options in Chapter 5.
■ Night Portrait. Combines flash with ambient light to produce an image that is mainly illuminated by the flash, but the background is exposed by the available light. This mode uses longer exposures, so a tripod, monopod, or IS lens is a must.
■ Flash Off. Absolutely prevents the built-in or external flash from firing, which you might want in some situations, such as religious ceremonies, museums, classical music concerts, and your double-naught spy activities.
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