The Human Eye and Defects of Vision

The eye as an optical system

Light entering the eye is refracted first at the curved front surface, the cornea, and then by the eye lens behind the iris. The combined system behaves as a single converging (convex) lens of short focal length that forms a real, inverted, diminished image on the retina at the back of the eyeball. The brain interprets this inverted retinal image as erect. The aperture of the system — the pupil — widens or narrows to control the amount of light admitted, much as the stop of a camera does.

Unlike a camera, where focusing is achieved by moving the lens toward or away from the film, the eye keeps the lens-to-retina distance fixed. Sharp focus for objects at different distances is instead obtained by changing the focal length of the eye lens itself. This adjustment is the defining feature of the living eye and is the basis of every defect discussed below.

Accommodation, near and far point

The ability of the eye lens to change its focal length so that objects at varying distances are imaged sharply on the retina is called accommodation. Ciliary muscles adjust the curvature of the eye lens: for a distant object the muscles relax and the lens is least curved (longest focal length), while for a near object the muscles contract, increasing the curvature and shortening the focal length. The state in which the relaxed eye views a distant object involves least strain, which is why an image formed for the relaxed eye is considered the most comfortable viewing condition.

Two distances bound the range of accommodation. The far point is the farthest distance the eye can see clearly; for a normal eye it lies at infinity. The near point is the closest distance at which the image of an object is formed by the eye lens on the retina. The NIOS lesson notes that the near point varies with the individual and with age — as close as 7–8 cm in a young child below ten years, and shifting to 100–200 cm or more in old age. The least distance of distinct vision, the closest distance at which the eye can see an object clearly without strain, is generally taken to be 25 cm for a normal eye and is denoted by $D$.

A defect of vision is, in optical terms, a shift of the near point or the far point away from these normal values, together with an inability of accommodation to compensate. Correction restores clear vision by placing an auxiliary lens in front of the eye so that the eye's own range is again usable.

Power of a lens and the dioptre

Spectacle prescriptions are quoted not as focal lengths but as a positive or negative number — the power of the lens. The NIOS lesson defines the power of a lens as the reciprocal of its focal length in metres:

$$ P = \frac{1}{f} $$

The SI unit of power is $\text{m}^{-1}$; the commercial unit used by opticians is the dioptre (D). The power of a convex lens is positive and that of a concave lens is negative, and a greater power implies a smaller focal length. A prescription of $+2\,\text{D}$ therefore denotes a convex lens of focal length $0.5\,\text{m}$, while $-2\,\text{D}$ denotes a concave lens of the same focal length. This single convention — positive for converging, negative for diverging — is what links the abstract lens to the corrective spectacle.

Myopia (short-sightedness)

In myopia, or short-sightedness, the eye sees nearby objects clearly but cannot focus distant ones. The eye lens converges light too strongly — either because the lens is too curved or the eyeball is elongated — so the image of a distant object forms in front of the retina. The far point of a myopic eye is no longer at infinity but at some finite distance; objects beyond that far point appear blurred.

Correction requires a concave (diverging) lens of negative power. The diverging lens spreads the parallel rays from a distant object so that they appear to originate from the eye's actual far point. The eye lens, working within its now-reduced range, then converges these rays onto the retina and the distant object is seen sharply.

Hypermetropia (long-sightedness)

Hypermetropia, or long-sightedness, is the reverse situation: the eye sees distant objects clearly but cannot focus nearby ones. The eye lens converges light too weakly — the lens is insufficiently curved or the eyeball is shortened — so the image of a near object would form behind the retina. The near point of a hypermetropic eye lies farther than the normal 25 cm, which is why such a person must hold reading material away from the face.

Correction requires a convex (converging) lens of positive power. The converging lens adds the convergence the eye lacks, so that an object held at the normal near point of 25 cm produces rays which the eye can finally focus on the retina. The auxiliary convex lens effectively forms a virtual image of the near object at the eye's own (more distant) near point.

Presbyopia and astigmatism

Presbyopia is the age-related loss of the power of accommodation. As the eye lens gradually loses its elasticity and the ciliary muscles weaken, the lens can no longer curve enough to focus on close objects, so the near point steadily recedes — the NIOS lesson records this shift to 100–200 cm or more in old age. Because an eye with presbyopia often struggles with both near and distant vision, it is commonly corrected with bifocal lenses, whose upper portion is concave (for distance) and lower portion convex (for reading).

Astigmatism arises when the cornea is not perfectly spherical, so its curvature differs along different planes. The eye then focuses light to different extents in those planes, and objects with lines in particular orientations appear blurred while others remain sharp. Astigmatism is corrected with a cylindrical lens whose curvature compensates for the uneven corneal surface in the affected plane.

Defect–correction summary

The table below consolidates each defect with its optical cause and the corrective lens. The two associations examiners test most are the lens type and the sign of its power, both of which follow directly from where the uncorrected image forms relative to the retina.