peak contrast sensitivity is at about four cycles per degree of visual angle. At higher spatial frequencies, comprising finer and finer lines, contrast sensitivity decreases, until at about forty cycles per degree even the brightest of bright lines and the darkest of dark lines cannot be seen. The high spatial frequencies in an image give it fine details. Reducing the contrast of an image reduces the visibility of these high spatial frequencies because contrast sensitivity for them already is poor. This is how a reduction of contrast can reduce the clarity of an image—by removing its fine details. It is important to emphasize that reducing the contrast is not the same as blurring an image. Blurring is accomplished by reducing the contrast only of the high spatial frequencies. Aerial perspective reduces the contrast of all spatial frequencies. In art In art, especially painting, aerial perspective, or atmospheric perspective, refers to the technique of creating an illusion of depth by depicting distant objects as paler, less detailed, and usually bluer than near objects. This technique was introduced in painting by Leonardo da Vinci to portray what was observed in nature and evident in his interest in optics. The technique allows a painter to capture the effect that atmosphere has on the appearance of an object as viewed from a distance. As the distance between an object and a viewer increases, the contrast between the object and its background decreases, and the contrast of any markings or details within the object also decreases. The colours of the object also become less saturated and shift toward the colour of the atmosphere, which is bluish when sunlit, but will shift to other colours under certain conditions (for instance, reddish around sunrise and sunset or saturated during fog). (One caution: in common speech, the words perspective and viewpoint tend to be used interchangeably; however, in art, aerial perspective does not imply an aerial viewpoint, such as that forming the basis of the aerial landscape genre. The example by Frans Koppelaar pictured here shows the difference. This landscape is a good example of aerial perspective; however, it is not an aerial landscape, since apparently, the observer is standing on the ground.) As such, the term atmospheric perspective can be understood to better describe how properties of the scene's atmosphere effect the appearance of an object as it moves further from the viewer. See also Aerial shot