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 and thickness (contributors to image size and aniseikonia) 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 (inducing effective refraction changes), as well as problems caused by oblique (marginal) astigmatism and chromatic aberration (if lens material remains the same). 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. This may be used to determine the amount of visual reduction that is caused by irregular astigmatism. Overrefraction can also be used in the retinoscopic examination of children.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.