What is Image Noise?

Image noise is the term applied to pictures, a counterpart to the white noise you would hear in an audio or video file. Analogue cameras will show image noise through grainy specs on the picture whereas digital cameras will show image noise through random speckles throughout the picture. While most of the time image noise should be avoided, sometimes it can create the illusion of an older picture. Things such as exposure, temperature and different camera modes can affect image noise. No matter what you do to prevent image noise, some will always be present. Any electronic unit that sends or receives a signal will be susceptible to it. For digital cameras, light that enters the lens and misaligns with the sensors will cause image noise. Even if you cannot see the noise when you look at your picture, there is some form of image noise in any image you take. The same thing can be said for audio and video productions. Every type of electronic device receives some sort of noise and sends it on to what it is creating.

The signal-to-noise ratio (SNR) is a way of comparing signal to noise for any electronic unit. High ratios have more visible image noise whereas low ratios will have non-visible image noise. These ratios are affected by the camera's ISO speed. The ISO speed refers to the sensitivity to light. They usually come in factors of two, such as ISO 50 or ISO 200. Higher numbers allow for a greater sensitivity to light. For example, a photo taking at an ISO 200 will take half as long to reach exposure as a photo taking at ISO 100 as long as all other settings are the same. Digital cameras can take pictures at different ISO speeds by amplifying image signal settings. However, this will increase the amount of image noise in pictures taken as you use higher ISO speeds.

There are three primary types of noise: Random, fixed pattern and banding. Random noise revolves around an increase in intensity of the picture. It occurs through color discrepancies above and below where the intensity changes. It is random, because even if the same settings are used, the noise occurs randomly throughout the image. It is generally affected by exposure length and ISO speed. Random noise is the hardest to get rid of because you cannot predict where it will occur. The digital camera itself cannot account for it, and it has to be lessened in an image editing program.

Fixed pattern noise surround hot pixels. Hot pixels are pixel bits that are more intense than others surrounding it and are much brighter than random noise fluctuations. Long exposures and high temperatures cause fixed pattern noise to appear. If pictures are taken under the same settings, the hot pixels will occur in the same place time and time again. Fixed pattern noise is the easiest type to fix after the fact. Once a digital camera realizes the fixed pattern, it can be adjusted to lessen the affects on the image. However, it can be more dubious to the eye than random noise if not lessened.

Banding noise really depends on the camera as not all digital cameras will create it. During the digital processing steps, the digital camera takes the data being produced from the sensor and creates the noise from that. High ISO speeds, shadows and photo brightening will create banding noise.

While you cannot eliminate image noise, you can take a few steps to reduce it if it is affecting your pictures. Generally, pictures taken at night are the most affected by image noise. Since darker pictures rely on longer exposure times or higher ISO speeds, they are more susceptible to bright pixel changes. The sensor intensifies the light taken in and bright dots will appear throughout the image in an effort to compensate during processing. Using a flash can prevent such noise. While most photographers would rather use a natural light source, sometimes this just is not possible. Flashes are often cheap, inexpensive and even the most advanced photographer can benefit from a professional flash. This will give you the light source you need to take clearer, crisper pictures and avoid brighter image noise showing up in pictures taken at night or in other low light situations. Corrective filters can also be used for reducing image noise for movement-based photos. Different filters can have multiple affects, reducing image noise and creating an even easier way for what noise might show up to be edited in a program after the fact.

Image noise is something every photographer should be aware of. While it can cause problems, it can also be solved with a little work. Digital cameras have settings that will reduce noise, but photos can also be edited after the fact. Image editing programs allow for you to add and reduce noise, but this does not always work. By taking steps to prevent as much noise as you can before you edit the program, you will eliminate the headache of a good picture being destroyed by some form of image noise. Even if you cannot avoid it, you can prevent it from ruining your photos to the best of your ability.