Contrast (vision)

Contrast is the contradiction in luminance or colour that makes an object (or its representation in an image or display) distinguishable. In visual perception of the real world, contrast is determined by the difference in the colour and brightness of the object and other objects within the same field of view. The human visual system is more sensitive to contrast than absolute luminance; we can perceive the world similarly regardless of the huge changes in illumination over the day or from place to place. The maximum contrast of an image is the contrast ratio or dynamic range. Images with a contrast ratio close to their medium's maximum possible contrast ratio experience a conservation of contrast, wherein any increase in contrast in some parts of the image must necessarily result in a decrease in contrast elsewhere. Brightening an image will increase contrast in dark areas but decrease contrast in bright areas, while darkening the image will have the opposite effect. Bleach bypass destroys contrast in both the darkest and brightest parts of an image while enhancing luminance contrast in areas of intermediate brightness. According to Campbell and Robson (1968), the human contrast sensitivity function shows a typical band-pass filter shape peaking at around 4 cycles per degree, with sensitivity dropping off either side of the peak. That finding has led many to claim that the human visual system is most sensitive in detecting contrast differences occurring at 4 cycles per degree. However, the claim of frequency sensitivity is problematic given, for example, that changes of distance do not seem to affect the relevant perceptual patterns (as noted, for example, in the figure caption to Solomon and Pelli (1994)). While the latter authors are referring specifically to letters, they make no objective distinction between these and other shapes. The relative insensitivity of contrast effects to distance (and thus spatial frequency) may also be observed by casual inspection of a paradigmatic sweep grating, as may be observed here The high-frequency cut-off represents the optical limitations of the visual system's ability to resolve detail and is typically about 60 cycles per degree.