The Correcting Lens
When the examiner uses the appropriate correcting lenses (with either loose lenses or a phoropter), the retinoscopic reflex is neutralized. In other words, when the examiner brings the patient’s far point to the peephole, the reflex fills the patient’s entire pupil (Fig 4-8). The power of the correcting lens (or lenses) neutralizing the reflex is determined by the refractive error of the eye and the distance of the examiner from the eye (the working distance). Theoretically, the working distance should be at optical infinity but this does not practically allow for changing lenses in front of the eye or seeing the retinal reflex. The dioptric equivalent of the working distance (ie, the inverse of the distance) must be subtracted from the power of the correcting lens to determine the actual refractive error of the patient’s eye. Common working distances are 67 cm (1.50 D) and 50 cm (2.00 D), and many phoropters have a 1.50 D or 2.00 D “working-distance lens” for use during retinoscopy, which are then removed at the end of the retinoscopy (however, this lens can produce bothersome reflexes). If the examiner is not using the “built-in” working lens in the phoropter, he or she must algebraically subtract the appropriate amount of spherical power to move the neutralization point from the examiner to infinity (Clinical Example 4-2).
Figure 4-7 Characteristics of the moving retinal reflex on both sides of neutrality. The vertical arrows indicate the position of the retinoscope with regard to the point of neutrality.
(Illustration by C. H. Wooley.)
Figure 4-8 Observation system at neutralization.
For example, an examiner obtains neutralization with a total of +4.00 D over the eye (gross retinoscopy) at a working distance of 67 cm. Subtracting 1.50 D for the working distance yields a refractive correction of +2.50 D.
Any working distance may be used. If the examiner prefers to move closer to the patient for a brighter reflex, the working-distance correction is adjusted accordingly. Working without an explicit lens to correct for the working distance may allow the use of fewer lenses held in front of the eye, reducing distracting reflections from the lens surfaces.
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.