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, and the contrast of any markings or details within the object also decreases. The colours of the object also become less saturated and shift towards the background color, which is usually blue, but under some conditions may be some other color (for example, at sunrise or sunset distant colors may shift towards red).
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. In such early paintings the distant objects tend to be represented as gray-green.
The major component affecting the appearance of objects during daylight is scattering of light, called skylight, 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 (see haze). 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.
The ability a person with normal visual acuity to see fine details is determined by his or her contrast sensitivity. Contrast sensitivity is the reciprocal of the smallest contrast for which a person can see a sine-wave grating. A person's contrast sensitivity function is contrast sensitivity as a function of spatial frequency. Normally, peak contrast sensitivity is at about 4 cycles per degree of visual angle. At higher spatial frequencies, comprising finer and finer lines, contrast sensitivity decreases, until at about 40 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 its fine details. Reducing the contrast of an image reduces the visibility of these high spatial frequencies because contrast sensitivity for them is already 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, especially painting, aerial 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.
(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 the observer is apparently standing on the ground.)