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Lens Aberrations 57

Refraction by the human eye is also subject to chromatic aberration. As a
result, the focusing power of the eye is different for different wavelengths
of light in the real world of polychromatic objects. Consequently there is
no place in the visual space where one can place an object and expect it to
be well focused on the retina for more than one wavelengths of light at a
time. Hence, blue wavelength focuses before the retina and red beyond the
retina, i.e. the far point of an individuals focus varies with the wavelengths
of light. In terms of foveal vision, the dominant chromatic aberration is
axial or longitudinal chromatic aberration.
The causes of chromatic aberration are dispersion in the cornea, aqueous,
crystalline lens and vitreous humor. Dispersion is simply a variation in the
refractive index of the material with various wavelengths of light and causes
white light to be dispersed into the various spectral colours, just as prism
disperses light into a rainbow. Refractive surgery techniques can not correct
chromatic aberration as this error is inherent to the properties of ocular
structures and not to the shape of the ocular components.

Influencing Factors
Spectral sensitivity of the eyes helps reduce the effect of chromatic
aberration by making the visual system more sensitive to the green
wavelength focused onto the retina.

Clinical Application
In clinical practice ocular chromatic aberration is made use of in Duo chrome
test.

SPHERICAL ABERRATION
Spherical aberration is an axial and wide beam aberration. The light rays
from the peripheral edge of the lens are refracted to a greater degree than
the light rays passing through the center of the lens. Peripheral rays bend
more than the paraxial rays. This creates a slight blurring of the image that
is minimized by the size of the lens.

Fig. 6.5: Spherical aberration