Visual acuity

Visual acuity is a measure of the ability of the eye to discriminate between two points. It is the central vision required for seeing details at all distances.

It is a function of the macular area of the retina, and in particular the central fovea, and is mediated by the retinal cones and their central connections.

Peripheral vision

The field of vision is that portion of space which can be seen by the eye. It is bounded medially by the nose, superiorly by the upper lid or brow and below by the cheek. An area of absent or depressed vision within this field is known as a scotoma. The optic nerve head (optic disc) has no visual receptors and thus results in an absolute scotoma temporal to fixation—the blind spot.

The visual field and defects within it can be measured by plotting the light threshold of different areas of the retina with static lights of varying intensity (static perimetry) or by moving targets of known size and luminance across the field (kinetic perimetry). Accurate charting of the visual fields requires instruments of varying complexity but simple diagnostic assessment is possible with confrontation methods.

Both of these methods will quickly identify gross field loss such as a homonymous or bitemporal hemianopia. With practise small central and paracentral field defects can be detected by method B.

Colour vision

Normal colour vision is required for certain occupations, for example certain branches of the armed forces and electrical engineering. About 7% of men and 0.5% of women are congenitally colour blind. The defect is usually in red/green differentiation and is hereditary, transmitted as a sex-linked abnormality.

Colour vision is most easily tested with pseudoisochromatic colour plates such as those of Ishihara and Hardy, Rand & Rittler. Acquired colour defects may be found in macular and optic nerve disease.

Pinhole effect.

Emmetropia.

Emmetropia (the optically normal eye)

Light from infinity is focused onto the retina without accommodation of the lens. Objects nearer than infinity are brought to a focus by accommodation. This is achieved by contraction of the ciliary muscle which slackens the zonule and allows the normal elasticity of the lens to increase its converging power.

Thus, an emmetropic eye does not require any correcting spectacle lens until the power of accommodation is inadequate for reading (presbyopia). This leads to the common situation of the middle aged who find that although distance vision remains satisfactory close vision becomes progressively worse, necessitating the wearing of reading glasses.

Myopia (short sight)

In this state light from infinity is focused in front of the retina producing a blurred image but near objects may be focused satisfactorily without accommodation.

Accommodation increases the converging power of the lens making myopia worse. The reverse process is not possible and myopia can only be corrected by a diverging (concave) lens.

Hypermetropia

In this condition light from infinity is focused behind the retina unless accommodation can correct this. In youth, when the range of accommodation is considerable, the hypermetrope may be able to overcome his refractive error but eventually a converging lens (convex) is required.

Astigmatism

The major refracting surface of the eye is the cornea. If this is shaped as part of a perfect sphere a point of light will be focused as such on the retina. However, if the corneal surface is aspherical then a point of light is focused as a smudge (astigmatism means ‘not a point’).

In the majority of cases the maximum and minimum curvatures of the cornea are at right angles to each other (regular astigmatism) and the optical defect can be corrected by the use of cylindrical lenses which refract light in one axis only. In certain pathological conditions, e.g. keratoconus and corneal scarring, the angle between the maximum and minimum curvatures is less than a right angle (irregular astigmatism) and the defect can only be corrected with a contact lens or corneal graft which provide a new spherical...