Using a Stenopaic slit, with the slit placed at 90 degrees the best sphere is -1.25D, when the slit is placed at 180 degrees the best sphere is -0.25D. What is the patient’s refractive error?

• A. -0.25 -1.00 x 180
• B. -1.25 x 180
• C. -0.50 -0.50 x 90
• D. Plano -1.00 x 180

Answer: (A)

Stenopaic slit testing reveals meridional powers: the meridian aligned with the slit shows the spherical component, while the meridian 90 degrees away includes the cylinder effect (with the axis as the meridian where the cylinder has no influence).

Here, when the slit is placed at 180 degrees, the best sphere is -0.25 D. That means along the 180° meridian the refractive power is purely the sphere, so the spherical component is -0.25 D and the cylinder’s axis is at 180°.

When the slit is at 90 degrees, the best sphere is -1.25 D, which equals the spherical component (-0.25) plus the cylinder contribution. The difference (-1.25 minus -0.25) is -1.00 D, so the cylinder power is 1.00 D with an axis at 180° (perpendicular meridian shows the full cylinder effect).

Therefore, the patient’s refractive error is -0.25 sphere with -1.00 cylinder at 180 degrees.