require cycloplegic refraction because of their high amplitude of accommodation. For more details on the cycloplegic drugs used in adults and children, please refer to the pharmacotherapeutics chapter in BCSC Section 2, Fundamentals and Principles of Ophthalmology.
Overrefraction
Phoropters may be used to refract the eyes of patients with highly ametropic vision. Variability in the vertex distance of the refraction (the distance from the back surface of the spectacle lens to the cornea) and other induced errors make prescribing directly from the phoropter findings unreliable.
Some of these problems can be avoided if highly ametropic eyes are refracted over the patients’ current glasses (overrefraction). If the new lenses are prescribed with the same base curve as the current lenses and are fitted in the same frames, many potential difficulties can be circumvented, including vertex distance error and pantoscopic tilt error, as well as problems caused by marginal astigmatism and chromatic aberration. Overrefraction may be performed with loose lenses (using trial lens clips such as Halberg trial clips), with a standard phoropter in front of the patient’s glasses, or with some automated refracting instruments.
If the patient is wearing spherical lenses, the new prescription is easy to calculate by combining the current spherical correction with the spherocylindrical overrefraction. If the current lenses are spherocylindrical and the cylinder axis of the overrefraction is not at 0° or 90° to the present correction, other methods previously discussed are used to determine the resultant refraction. Such lens combinations were often determined with a lensmeter used to read the resultant lens power through the combinations of the old glasses and the overrefraction correction. This procedure is awkward and prone to error because the lenses may rotate with respect to one another on transfer to the lensmeter. Manual calculation is possible but complicated. Programmable calculators can be used to perform the trigonometric combination of cylinders at oblique axes, but they may not be readily available in the clinic.
Overrefraction has other uses. For example, a patient wearing a soft toric contact lens may undergo overrefraction for the purpose of ordering new lenses. An overrefraction is especially useful for patients wearing rigid, gas-permeable, hard contact lenses for irregular corneal astigmatism or corneal transplants. Overrefraction can also be used in the retinoscopic examination of children.
Spectacle Correction of Ametropias
Ametropia is a refractive error; it is the absence of emmetropia. The most common method of correcting refractive error is through prescription of spectacle lenses.
Spherical Correcting Lenses and the Far Point Concept
The far point plane of the nonaccommodated eye is conjugate with the retina. For a simple lens, distant objects (those at optical infinity) come into sharp focus at the secondary focal point (F2) of the lens. To correct the refractive error of an eye, a correcting lens must place the image it forms (or its F2) at the eye’s far point. The image at the far point plane becomes the object that is focused onto the retina. For example, in a myopic eye, the far point lies somewhere in front of the eye, between it and optical infinity. In this case, the correct diverging lens forms a virtual image of distant objects at
its F2, coincident with the far point of the eye (Fig 3-20).
Figure 3-20 A diverging lens is used to correct myopia.
The same principle holds for the correction of hyperopia (Fig 3-21). However, because the far point plane of a hyperopic eye is behind the retina, a converging lens must be chosen in the appropriate power to focus parallel rays of light to the far point plane.
Figure 3-21 A converging lens is used to correct hyperopia.
The Importance of Vertex Distance
For any spherical correcting lens, the distance from the lens to its focal point is constant. Changing the position of the correcting lens relative to the eye also changes the relationship between the F2 of the correcting lens and the far point plane of the eye. With high-power lenses, as used in the spectacle correction of aphakia or high myopia, a small change in the placement of the lens produces considerable blurring of vision unless the lens power is altered to compensate for the new lens position.
With refractive errors greater than ±5.00 D, the vertex distance must be accounted for in prescribing the power of the spectacle lens. A distometer (also called vertexometer) is used to measure the distance from the back surface of the spectacle lens to the cornea with the eyelid closed (Fig 3-22). Moving a correcting lens closer to the eye—whether the lens has plus or minus power— reduces its effective focusing power (the image moves posteriorly away from the fovea), whereas moving it farther from the eye increases its focusing power (the image moves anteriorly away from
the fovea).
