Optical system
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Optical Characteristics
The optics of the eye are not perfect. There is measurable blurring of the retinal
image even under ideal viewing conditions, and the visual system is subject to
spherical and chromatic aberrations.
Edge spread for the human eye with a 3 mm pupil (Gubisch,
1967).
Figure 10 shows the measured distribution of light across the retina when
viewing a sharp edge between luminances of two different levels. The blur of
the edge is several minutes of arc wide.
Visual objects are normalized to the distance from which they are viewed.
The size of an object is described by the angle formed when lines are drawn
from the eye to the edges of the object. The angle is measured in arc minutes
(1/60 of an arc degree) when objects are near the resolving power of the eye.
Different wavelengths come into focus at different distances behind the lens
of the eye. This is called chromatic aberration.
Chromatic aberration of the eye. The zero point of
the ordinate is arbitrary (587 nm).
Figure 11 shows the relative refractive differences of various wavelengths.
The zero point of the ordinate, at 587 nanometers, is arbitrary. 587nm is the
Fraunhofer d line, the spectral emission of helium.
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