Refraction and Corrective Lenses
What the exam tests
Refraction and Corrective Lenses tests your ability to classify ametropia, interpret retinoscopy and subjective findings, and write or transpose a spectacle prescription accurately.
Ametropia
In myopia, parallel rays focus in front of the retina; correction is with a minus lens. In hyperopia, rays would focus behind the retina with accommodation relaxed; correction is with a plus lens. Regular astigmatism arises when the two principal meridians have different powers; a cylinder corrects it, and the axis states the meridian of zero added power. With-the-rule astigmatism, common in young people, has the vertical meridian steeper. Presbyopia is loss of accommodative amplitude with age and is corrected with a plus near addition.
Prescription format and transposition
A prescription records sphere, cylinder, axis, add, prism and pupillary distance for each eye. To transpose, add sphere and cylinder to get the new sphere, reverse the cylinder sign, and rotate the axis by 90 degrees. Spherical equivalent equals sphere plus half the cylinder; it is a summary value, not a substitute prescription. Vertex distance matters for powers above about ±4 D.
Worked example
Transpose +2.00/−1.00 × 180. New sphere = +2.00 + (−1.00) = +1.00. New cylinder = +1.00. Axis = 180 − 90 = 90. Result: +1.00/+1.00 × 90. Both forms describe the same lens.
Traps
Do not rotate the axis without changing the cylinder sign, write a cylinder without an axis, or treat 180 and 0 as different axes. Avoid using the spherical equivalent as the final prescription, and remember that retinoscopy is objective while subjective refraction needs patient responses.