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5. As the shots are taken, three indicators will appear on the exposure scale in the viewfinder, with one of them flashing for each bracketed photo, showing when the base exposure, underexposure, and overexposure are taken.

6. Bracketing remains in effect when the set is taken so you can continue shooting bracketed exposures until you use the electronic flash, or return to the menu to cancel bracketing.

Figure 4.12 shows a bracketed series of shots.

Dealing with Noise

Image noise is that random grainy effect that some like to use as a visual effect, but which, most of the time, is objectionable because it robs your image of detail even as it adds that "interesting" texture. Noise is caused by two different phenomena: high ISO settings and long exposures.

High ISO noise commonly appears when you raise your camera's sensitivity setting above ISO 400. With Canon cameras, which are renowned for their good ISO noise characteristics, noise may become visible at ISO 800, and is usually fairly noticeable at ISO 1600. This kind of noise appears as a result of the amplification needed to increase the sensitivity of the sensor. While higher ISOs do pull details out of dark areas, they also amplify non-signal information randomly, creating noise.

Figure 4.12 Bracketing allows shooting a series of shots at different exposures automatically.

A similar noisy phenomenon occurs during long time exposures, which allow more photons to reach the sensor, increasing your ability to capture a picture under low-light conditions. However, the longer exposures also increase the likelihood that some pixels will register random phantom photons, often because the longer an imager is "hot" the warmer it gets, and that heat can be mistaken for photons.

There's also a special kind of noise that CMOS sensors like the one used in the XTi are potentially susceptible to. With a CCD, the entire signal is conveyed off the chip and funneled through a single amplifier and analog to digital conversion circuit. Any noise introduced there is, at least, consistent. CMOS imagers, on the other hand, contain millions of individual amplifiers and A/D converters, all working in unison. Because all these circuits don't necessarily all process in precisely the same way all the time, they can introduce something called fixed-pattern noise into the image data.

Fortunately, Canon's electronics geniuses have done an exceptional job minimizing noise from all causes in the XTi. Even so, you might still want to apply the optional long exposure noise reduction that can be activated using Custom Function 02. This type of noise reduction involves the XTi taking a second, blank exposure, and comparing the random pixels in that image with the photograph you just took. Pixels that coincide in the two represent noise and can safely be suppressed. This noise reduction system, called dark frame subtraction, effectively doubles the amount of time required to take a picture, and it is used only for exposures longer than one second. Noise reduction can reduce the amount of detail in your picture, as some image information may be removed along with the noise. So, you might want to use this feature with moderation.

To activate your XTi's long exposure noise reduction feature, go to the Custom Function Menu, choose C.Fn-02, shown in Figure 4.13, and press the Set button. You can then use the up/down cross keys to scroll among the three options (0 through 2), originally described in Chapter 3, but recapped here:

■ 0: Off. Disables long exposure noise reduction.

■ 1: Auto. Applies noise reduction for exposures longer than 1 second if the camera detects long exposure noise.

■ 2: On. When this setting is activated, the XTi applies noise reduction to all exposures longer than 1 second.

You can also apply noise reduction to a lesser extent using Photoshop, and when converting RAW files to some other format, using your favorite RAW converter or an industrial-strength product like Noise Ninja (www.picturecode.com) to wipe out noise after you've already taken the picture.

Figure 4.13 Long exposure noise reduction can reduce random pixels produced at longer shutter speeds.

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