Optics, Refraction, and Low Vision
Core concepts
Understand refractive error: myopia (minus lenses), hyperopia (plus lenses), astigmatism (cylinder), and presbyopia (loss of accommodation). Know the optics of the eye: corneal power, lens power, and axial length. Master the difference between spherical equivalent and astigmatic correction. Know low vision aids: magnifiers, telescopes, and electronic devices, and the principles of magnification. Understand the pinhole test (improves refractive error, not media opacity) and the difference between refractive and organic vision loss.
Traps
Questions test that a pinhole improves vision in refractive error but not in cataract or retinal disease. Another trap: confusing spherical equivalent (sphere plus half cylinder) with the actual prescription. Also know that myopia increases risk of retinal detachment and glaucoma.
Worked example / rule of thumb
A patient has blurred vision that improves with a pinhole. That suggests uncorrected refractive error, not pathology. Rule of thumb: pinhole improvement equals refractive error; no improvement suggests organic disease.