NoiseNoise is a random variation of image density, visible as grain in film and pixel level variations in digital images. It arises from the effects of basic physics— the photon nature of light and the thermal energy of heat— inside image sensors. Noise and its measurement are introduced in Noise in photographic images. Noise is measured by several Imatest modules. Stepchart produces the most detailed results, but noise is also measured in Colorcheck, SFR, SFRplus, and Uniformity. |
Original | Noise added  |
Noise scales strongly with pixel area. It can be very low in digital SLRs, which have pixels at least 5 microns square. But it can get ugly in compact digital cameras with small sensors, especially at high ISO speeds. It is also affected by sensor technology and manufacturing quality.
Typical noise reduction (NR) software reduces the visibility of noise by smoothing the image, excluding areas near contrast boundaries. This technique works well, but it can obscure fine, low contrast detail. Some specialized programs, such as Neat Image, which is trained to recognize the structure of noise, have fewer side effects. Several of the patterns in Test Charts can be printed and photographed (or entered directly into software) to examine the effects of NR algorithms.
