Artifacts in Digital Images

Artifacts in digital images are unwelcome and unnatural elements or distortions. While they abound in reproductions of all kinds they often go unnoticed, perhaps because we are so used to them. Problems that we perceive in images are often the result of several causes combined, but let's try to look at some of the more prominent ones individually.

Pixelation due to too few sensor points being used to describe too big a picture. Note the stepping effect most clearly seen on the diagonal lines in the legs. We are all familiar with the idea of noise in sound but the same concept can be applied in the visual and electronic context. Charge Coupled Devices (CCDs) used in just about every digital camera and desktop scanner today, characteristically produce a number of different types of noise. Electronic fixed pattern and "dark" noise is present all the time in these devices, so just before an exposure is made, a digital camera will do what's known as a dark reading, to ascertain the signal from each sensor in its CCD array. When the actual exposure is made, the camera program then subtracts the dark noise signal from the exposure signal. This is one of the reasons why there is always a momentary delay between the time when you press the shutter button and when the picture is actually made.

The dark noise subtraction method is never perfect, particularly when the sensors are hot. For this reason some of the professional level and astronomical cameras use built-in refrigeration systems in order to keep the sensor cool. The signal to noise ratio is somewhat dependent on the "fill factor" of the pixel, or the percentage of the sensor element that gathers photons. Smaller elements have a higher inherent noise ratio and a very low signal to noise ratio or a very weak signal leads to problems that are hard to solve. Noise is also introduced during the analog to digital (A/D) conversion of the image data. Much research has been done in the field of CCD noise suppression, and a lot of progress has been made so that modern CCD cameras and scanners are much better than their predecessors.

CCD sensors have a bias toward the red and infrared end of the energy spectrum. If you look at the sensor you will usually see a bluish filter or filtration layer over it, designed to mask out sensitivity to unwanted infrared wave lengths, and thereby increase the signal from the blue end of the visible spectrum. Because of insensitivity at the blue end it's in the blue channel that you will almost certainly find the most noise in a digital photograph or scan. In some cases it can be helpful to open the blue channel in software such as Adobe Photoshop and apply a moderate amount of softening.

Expect more noise to appear in situations with low light levels where the noise to signal ratio will be at its highest. Perhaps surprisingly, a lack of noise can also be a problem. One reason that computer generated images such as those from 3D rendering programs, look so artificial is that they lack noise. Gradations are perfect in tone and color and edges are unnaturally abrupt. Keep this in mind if you are photo retouching and applying gradations or tone fills. It pays to also add a little noise.