Contrast (vision)

Contrast ( "stand" from the Italian contrasto " contrast " to the Latin contra 'against' and stare borrowed ) is the difference between light and dark areas of an image ( it distinguishes light and dark colors ). Colloquially speaking also of brilliance that is technically not defined in contrast.

Definition

The contrast is a distinguishing feature of the brightness profile of an image or between two pixels. The contrast range or dynamics describe the difference in intensity between the brightest and the darkest point of an image. In the general case, the contrast in dependence upon the resolution of the modulation transfer function is described.

The contrast is defined by the maximum and minimum luminance levels and.

The Weber contrast as:

The Michelson contrast modulation Km respectively as:

While the human eye the contrast range occurring in nature copes with relative ease ( the intensities are perceived not linear but logarithmic, see Weber- Fechner law ), we encounter in the effort to capture the seen, to some sobering limits. Contrast phenomena can cause optical illusions, but are fine line structures also involved in the recognition. Examples of this are some of the " canals of Mars " and gray tones, the Mach stripes.

The loss of contrast due to vanishing differences in luminance () is called in the dark as blackout or in very bright lighting conditions as Whiteout.

Image representation

In analogue photography next to the camera lens depends above all the footage over the contrast, photo prints, in addition, the gradation of the photo paper. In digital imaging, there is the power of the analog-to- digital converter. Newer digital cameras try the dynamic range by a non- linear response ( similar to the human eye) to expand. An important factor influencing the contrast range is also the chosen ISO Speed ​​: Raise the ISO speed results in a rule to lower representable contrast range. Photographic aperture and shutter speed to move only the area, but not enlarge the scope.

Photos with particularly high subject contrast, give more bright parts of the image brighter and more dark parts of the image even darker turn again, as shown in the image. The image appears for the observer contrast (ie the opposite of " flat" ), the viewer but overall shows less detail, that is, in the dark (shadow ) and light areas ( highlights) is not a drawing ( tonal range ) more visible, it can therefore see any details. If a motif larger differences in brightness than the digital camera can capture, so the camera can the tonal range in the image is not completely portray. In this case, it may be cheaper, more just to illuminate the details in the highlights not to lose (eg the structure of sunlit clouds). Because blown-out lights can hardly be restored, while turned out too dark image areas are mostly to save by appropriate post yet.

In the digital post-processing of images, however, a high contrast ratio is better in any case, since the contrast subsequently increased within wide limits, but can be reduced again very limited.

One way of contrast manipulation of digital images offers the inclusion of bracketing in conjunction with the image editing by HDR software.

In the evaluation of lenses the contrast plays a crucial role. The modulation transfer function describes the course of the contrast decreases with increasing spatial frequency and thereby limits the resolution.

From the painting techniques of the so-called seven color contrasts are known to make contrast images.

Imaging techniques (medicine)

Often a contrast enhancement is necessary in the imaging techniques in medicine. In many cases, it takes place before the recordings by injection of a contrast agent into the blood stream, such as angiography or magnetic resonance imaging. Here, through series of images, the temporal distribution of the contrast agent - that is, the circulation of each organ or connective tissue - are tested. Another less finely tunable nature of the contrast medium is performed in diagnostic radiology of the gastro -intestinal tract ( eg by ingestion of barium sulfate suspensions) or in computed tomography by iodinated contrast agents.