Nikon D7000 manual Dealing with Noise

Chapter 4 ■ Getting the Right Exposure 119

ISO settings can, of course, also be used to boost or reduce sensitivity in particular shoot- ing situations. The D7000 can use ISO settings from ISO 100 up to ISO 6400, plus H

1.0and H 2.0 (ISO 12800 and 25600 equivalent). The camera can also adjust the ISO automatically as appropriate for various lighting conditions. When you choose the Auto ISO setting in the Shooting menu, as described in Chapter 8, the D7000 adjusts the sensitivity dynamically to suit the subject matter, based on minimum shutter speed and ISO limits you have prescribed. As I noted in Chapter 8, you should use Auto ISO cau- tiously if you don’t want the D7000 to use an ISO higher than you might otherwise have selected.

Dealing with Noise

Visual image noise is that random grainy effect that some like to use as a special 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 the Nikon D7000, noise may become visible at ISO 1600, and is often fairly noticeable at ISO 3200. At ISO 6400 and above, noise is usually quite bothersome. Nikon tips you off that ISO 12800 and 25600 may be tools used in spe- cial circumstances only by labeling them H1.0 and H2.0. You can expect noise and increase in contrast in any pictures taken at these lofty ratings. High ISO 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. You’ll find a High ISO NR choice in the Shooting menu, where you can specify High, Norm, or Low noise reduction, or turn the feature off entirely. Because noise reduction tends to soften the grainy look while robbing an image of detail, you may want to disable the feature if you’re willing to accept a little noise in exchange for more details.

A similar noisy phenomenon occurs during long time exposures, which allow more pho- tons to reach the sensor, increasing your ability to capture a picture under low light con- ditions. 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 D7000 are potentially susceptible to. With a CCD, the entire signal is conveyed off the chip and funneled through a sin- gle 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 these circuits don’t nec- essarily all process in precisely the same way all the time, they can introduce something called fixed-pattern noise into the image data.