Aerial perspective
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Aerial perspective or atmospheric perspective is the effect on the appearance of an object by the atmosphere between it and a viewer (or the technique of depicting this effect in a work of art, such as a landscape painting). As the distance between an object and a viewer increases, the contrast between the object and its background decreases. The contrast of any markings or details on the object also decreases. The colours of the object also become less saturated and shift towards blue.
Aerial perspective was discovered and named by Leonardo Da Vinci, who used it in many of his works, such as the Mona Lisa, in order to suggest distance.
The major component affecting the appearance of objects during daylight is scattering of light, called sky light, into the line of sight of the viewer. Scattering occurs from molecules of the air and also from larger particles in the atmosphere such as water vapor and smoke. Scattering adds the sky light as a veiling luminance onto the light from the object, reducing its contrast with the background sky light. Skylight usually contains more short wavelength light than other wavelengths (this is why the sky usually appears blue), which is why distant objects appear bluish. A minor component is scattering of light out of the line of sight of the viewer. Under daylight, this either augments the contrast loss (e.g., for white objects) or opposes it (for dark objects). At night there is effectively no skylight (unless the moon is very bright), so scattering out of the line of sight becomes the major component affecting the appearance of self-luminous objects. Such objects have their contrasts reduced with the dark background, and their colours are shifted towards red.
It is important to emphasize that aerial perspective does not blur the outlines or the markings of objects. Objects appear less distinct because of the reduction in contrast. To illustrate that the air does not blur objects, one can look at the edge of the moon when it is on the horizon. Despite the light path being the longest possible through the atmosphere, the edge of the moon looks sharp.
In art, aerial perspective is used to describe the painting technique of creating depth by depicting distant objects as paler, less detailed, and bluer than near objects.
(One caution: in common speech, the words perspective and viewpoint tend to be used interchangeably; however, in art, aerial perpective 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 the observer is apparently standing on the ground.)
