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Lens Aberrations 55

Ideally, the off-axis performance of the lens should be the same as its
performance at the optical centre. But it is never so. The off-axis images are
afflicted with various types of aberrations, which spoil the quality of image
formed by the lens. There are six major lens aberrations that work against
obtaining a perfect image through the periphery of the lens. They are:

CHROMATIC ABERRATION

Fig. 6.2: Axial chromatic aberration
Chromatic aberration is a defect in the lens in which the various colours
of the spectrum are not brought to the same focus. Blue light is refracted
more than the red light when it passes through a lens. The result is out of
focus image. The wearer complains of peripheral colour fringes around
the object which is more pronounced off-axis. The higher the power of the
lens—the greater is the chromatic aberration.
Chromatic aberration depends upon the material of the lens. Since, the
lens materials have a different refractive index for each wavelength—the
lens will have a different focal length for each wavelength. The refractive
index is larger for blue than the red wavelength, so focal length is less for
blue than the red.

Importance
Since chromatic aberration occurs because the refractive index of the lens
material varies with the wavelength of the incident light, it gives rise to
what is called the Abbe value of the lens material which is denoted by V-
value. Higher Abbe value implies low chromatic aberrations and vice versa.
So polycarbonate lens with Abbe value of 30 causes more chromatic
aberration than CR lens with Abbe value of 58.
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Correction

1. The easiest solution to minimize chromatic aberration is to change the
lens material to higher abbe value.
2. Careful placement of optical centre with monocular pupillary distance
and its height in a small frame may reduce the chromatic aberration.