Visual image noise is the random grainy effect on the photograph which is disturbing most of the time rather than pleasing. If these grainy dots, also known as a digital noise, are noticeable or objectionable in the photograph, it can ruin the beauty of the photograph by distracting the viewers mind from the subject. Digital noise is mainly caused by either high ISO settings or a long exposure shooting. Let's discuss these two main factors which contributes to the production of the digital noise.

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 the lights. 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 the details out of the dark areas, they also amplify non-signal information randomly, creating the 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. Enabling the noise reduction tends to soften the grainy look, 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 the 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 the noise and can safely be suppressed. This noise reduction system, also 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 details in your picture, as some image information may be removed along with the noise. So, you might want to use this feature with moderation.

If you shoot in a RAW mode, you can also apply the noise reduction to a lesser extent by using post processing software like Photoshop, Lightroom, CaptureNX2 or Aperture later on the computer.

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