Dealing with Digital Noise

Visual image noise is that random grainy effect which is disturbing to the photograph most of the time rather than pleasing effect. If these grainy dots in the photograph which we also called digital noise are noticeable or objectionable, it can ruin the beauty of photographs by distracting viewers mind from the subject. Digital noise is mainly caused by either high ISO setting or long exposure shooting.

1. High ISO Setting

High ISO noise commonly appears when you raise your camera’s sensitivity setting above ISO 400. Different camera’s sensitivity level with light differs from one another and sometimes it determines how good your camera is with lights but with the Nikon D90, noise may become visible at ISO 800, and is often fairly noticeable at ISO 1600. And at ISO 3200, noise is usually quite bothersome. Nikon advises that using ISO 6400 in special situations like very low light condition may help you but you should consider labeling it H1.0. You can expect noise in any pictures taken with that ISO level which is obvious. High ISO noise appears as a result of the amplification needed to increase the sensitivity of the sensor. While using higher ISO settings do pull details out of dark areas, they also amplify non-signal information randomly, creating noise. You’ll find a High ISO NR option in the Shooting menu, where you can specify High, Norm, or Low noise reduction, or turn the feature off entirely. As Noise reduction tends to soften the grainy look while running an image of detail, you may want to disable the feature if you‘re willing to accept a little noise to get more details in your picture.

2. Long Exposure Shooting

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 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 D90 are potentially susceptible to. With a CCD sensor, 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 call process in precisely the same way all the time, they can introduce something called fixed-pattern noise in the image data.

Fortunately, Nikon has done an exceptional job minimizing those noises from all causes in the D90 and other Nikon DSLRs. Even so, you might still want to apply the optional Long Exposure Noise Reduction that can be activated using “Long exp. NR” in the Shooting menu, where the feature can be turned On or Off. When the Long exp NR feature is turned on, D90 takes a second, blank exposure and compares the random pixels in that image with the photograph you just took (first image). Pixels that coincide in the two pictures 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 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.

You can also apply noise reduction to a lesser extent using post processing software like Photoshop, CaptureNX2 or Aperture when working with RAW files to wipe out noise after you’ve already taken the picture.

Source: David Busch’s Nikon D90 Guide to Digital SLR Photography

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