Optical aberrations

The eye, in common with many optical systems in practical use, is by no means optically perfect; the lapses from perfection are called aberrations.

Fortunately, the eyes possess those defects to so small a degree that, for functional purposes, their presence is immaterial. It has been said that despite imperfections the overall performance of the eye is little short of astonishing. Physiological optical defects in a normal eye include the following :

1. Diffraction of light.

Diffraction is a bending of light caused by the edge of an aperture or the rim of a lens. The actual pattern of a diffracted image point produced by a lens with a circular aperture or pupil is a series of concentric bright and dark rings. At the centre of the pattern is a bright spot known as the Airy disc.

2. Spherical aberrations.

Spherical aberrations occur owing to the fact that spherical lens refracts peripheral rays more strongly than paraxial rays which in the case of a convex lens brings the more peripheral rays to focus closer to the lens.

The human eye, having a power of about +60 D, was long thought to suffer from various amounts of spherical aberrations.

However, results from aberroscopy have revealed the fact that the dominant aberration of human eye is not spherical aberration but rather a coma-like aberration.

3. Chromatic aberrations.

Chromatic aberrations result owing to the fact that the index of refraction of any transparent medium varies with the wavelength of incident light. In human eye, which optically acts as a convex lens, blue light is focussed slightly in front of the red.

In other words, the emmetropic eye is in fact slightly hypermetropic for red rays and myopic for blue and green rays. This in fact forms the basis of bichrome test used in subjective refraction.

4. Decentring.

The cornea and lens surfaces alter the direction of incident light rays causing them to focus on the retina. Actually these surfaces are not centred on a common axis. The crystalline lens is usually slightly decentred and tipped with respect to the axis of the cornea and with respect to the visual axis of the eye.

It has been reported that the centre of curvature of cornea is situated about 0.25 mm below the axis of the lens. However, the effects of deviation are usually so small that they are functionally neglected.

5. Oblique aberration.

Objects in the peripheral field are seen by virtue of obliquely incident narrow pencil of rays which are limited by the pupil. Because of this, the refracted pencil shows oblique astigmatism.

6. Coma.

Different areas of the lens will form foci in planes other than the chief focus. This produces in the image plane a 'coma effect' from a point source of light.

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