Stenopeic Slit Technique
The stenopeic slit is an opaque trial lens with an oblong slit whose width forms a pinhole with respect to vergence perpendicular to the slit (Fig 4-16). If an examiner is unable to decipher the astigmatism by performing the usual retinoscopy because of the subject eye’s irregular astigmatism or unclear media, he or she may neutralize the refractive error with spherical lenses and the slit at various meridians to find a spherocylindrical correction. This correction can then be refined subjectively. This process is especially useful for patients with small pupils, lenticular or corneal opacities, and/or irregular astigmatism. If the subject can accommodate, fog and unfog using plus sphere to find the most plus power accepted. Then turn the slit until the subject says the image is sharpest. If, for example, −3.00 D sphere is best there, when the slit is oriented vertically, this finding indicates −3.00 D at 90° in a power cross. If the best sphere with the slit oriented horizontally is −5.00 D, then the result is −3.00 −2.00 × 90. It is helpful to think of the “axis” of the stenopeic slit as a thin line perpendicular to the orientation of the slit.
Figure 4-16 Stenopeic slit. The image on the right demonstrates the placement of a spherical lens in front of the stenopeic slit in order to determine the best visual acuity.
(Courtesy of Tommy Korn, MD.)
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.