Refraction and Clinical Optometric Science
This domain tests refractive error classification, objective and subjective refraction, astigmatism, presbyopia, and clinical optics. Accuracy with cylinder and working distance is essential.
Core rules
- Myopia: parallel rays focus anterior to the retina; corrected with minus lenses. Hyperopia: focus behind the retina; corrected with plus lenses. Astigmatism: unequal meridian powers.
- Spherical equivalent = sphere + one-half cylinder. It is useful for estimating the power of a spherical lens but does not correct cylinder.
- With-the-rule astigmatism has the steeper meridian vertical (near 90 degrees); against-the-rule has the steeper meridian horizontal (near 180 degrees).
- Retinoscopy finds neutrality; with motion suggests undercorrection with a plus lens, against motion suggests overcorrection. Subtract the working distance in diopters.
- Jackson cross cylinder refines cylinder axis and power; duochrome compares red and green focus, with red preferred in myopia and green in hyperopia.
Worked example
A refraction is -2.00 -1.00 x 180. Reasoning: the spherical equivalent is -2.00 + (-0.50) = -2.50 D. The minus cylinder axis is 180 degrees, so the steeper meridian is 90 degrees; this is with-the-rule astigmatism. The patient needs more minus power vertically than horizontally.
Traps that cost marks
- Forgetting to subtract the retinoscopy working distance; a 50 cm distance adds +2.00 D.
- Reversing the axis when converting plus and minus cylinder.
- Using spherical equivalent as a substitute for full astigmatic correction in symptomatic patients.