The Oil Painter's Bible - chapter 5
As objects recede in space they not only appear to shrink in size, but tend to lose detail, contrast of
values, intensity of color, and their edges appear less distinct the greater the distance from the viewer's
eyes. This is the principle of Atmospheric Perspective. Some writers call it "aerial perspective," but this is
misleading, as the term, "aerial" usually pertains to flying.
The visual alteration of images over
distance is the direct result of "X"
amount of atmosphere between the
eye and the object or plane in view.
The atmosphere contains water vapor
and its own density, which renders it
somewhat less than totally
transparent, adding a certain degree
of whiteness to the air. Light renders
the atmosphere white. The more air
we must look through to view
something, the more atmosphere we
see between it and our eyes, and the
more the image is altered by it.
An optical illusion is created by the
presence of a semitransparent white
between the eye and any color darker
than white, which alters the color in
question in the direction of blue, as
well as lightening the value. This is
precisely why the sky appears to be
blue. The sky's blue is created by the
blackness of space being viewed
through a layer of semitransparent
white atmosphere. The white is the
atmosphere illuminated by the sun.
At night, without the sun's light, the atmosphere is no longer white, and the blackness of space becomes
visible. Note that the sky is always lightest just above the horizon. This is the greatest distance we can see
at ground level, which is where the atmosphere contains the most (white) water vapor and the greatest
density. At the horizon, the density of the atmosphere renders it more opaque, and thus, whiter. As we
look up, we look through thinner air, which is less opaque, and the sky is bluer and darker. This is why
distant objects and planes appear lighter, bluer, and less distinct. The same phenomenon can be produced
with paint. The process is called scumbling and is accomplished by applying a thin veil of white paint
semitransparently over a layer of (dry) darker paint. The optical result in paint is the same as in the air.
Translucent white over black reads bluish, just as light grey smoke against dark trees reads blue. Note that
the same smoke may appear to be brown when a white cloud is behind it, a reverse of the scumble
phenomenon. Dark over light increases apparent warmth. This is the principle at work in glazing, that is,
the application of darker transparent paint over a lighter passage. Glazing and scumbling are discussed at
length in Chapter Six and Chapter Ten.
In painting, atmospheric perspective can be rendered directly, in one step, using opaque paint
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