Digital image noise shows up most prevalently as speckles in uniform tonal areas such as blue sky or open shadows. The effect looks like a monochromatic grain similar to film grain (luminance noise) and/or colored waves (color noise). Noise increases at higher temperatures. It also increases with sensitivity, less at 100 ISO, more at 1600 ISO. In addition, the smaller the imaging sensor pixels the greater noise level results. This is why compact digital cameras generate noisier images than full size digital SLRs. Professional grade digital cameras with their more advanced imaging sensors, more powerful processors and enhanced noise reduction algorithms display virtually no noise, especially at lower ISO settings.

The Cause of Digital Image Noise
Each pixel in a digital camera sensor contains one or more light sensitive photodiodes which convert the incoming light into an electrical signal which is processed into the color value of the pixel in the final image. These signals must be amplified which increases image information as well as any inherent 'background' signal which can be thought of as hiss like in audio equipment.  Even when you have no sound input to an amplifier there is a faint hiss which gets louder when you turn up the volume.

 If the same pixel would be exposed several times by the same amount of light, the resulting color values would not be identical but have small statistical variations, called "noise". Even without incoming light, the electrical activity of the sensor itself will generate some signal, the equivalent of the background hiss of audio equipment which is switched on without playing any music. This additional signal is "noisy" because it varies from pixel to pixel, and over time. Noise also increases with the temperature, adding to overall image noise. This inherent noise is called the "noise floor". The output of a pixel has to be greater than the noise floor to produce a coherent image.  For more information about Digital Noise check out what Jeff Medkeff has to say by Clicking Here.   He presents a very balanced, more technical and comprehensive discussion.

Long Exposure Noise
Another type of noise, often referred to as "stuck pixels" or "hot pixel" noise, occurs with long exposures (1-2 seconds or more) and appears as a pattern of colored dots (slightly larger than a single pixel). New generation digital cameras have overcome much of the long exposure noise problem.

The effect of long exposure stuck pixels can be reduced to a great extent by taking a "dark frame" (with lens cap on) either before or after the main shot and subtracting this from the original shot. Many newer digital cameras have built-in long exposure noise reduction and take a "dark frame" with the shutter closed for the same amount of time as the main image. This dark frame is then used to identify and subtract the "stuck pixels". But even with noise reduction off, newer cameras will show fewer stuck pixels than images taken with an older generation digital camera.

Minimizing Digital Image Noise

• Avoid slow or long exposures.
• Avoid using high ISO settings.
• Keep cameras from excessive heat.
• When comparing digital cameras, all things being equal, favor the one with the larger image sensor and good low-noise internal processing.
• Use noise reduction software. This will allow you to take pictures and not miss some great shots. You'll find that a good noise reduction software usually does a pretty good job of 'cleaning' up the noise to an acceptable level in most noisy images.