Ocular Anatomy, Physiology, and Refractive Conditions
Structures and their roles
The eye has two refracting elements: the air–cornea interface supplies roughly two-thirds of total power (about 43 D of the eye's ~60 D), and the crystalline lens adds variable power through accommodation. Know the anterior corneal radius (~7.8 mm), corneal index (1.376), the anterior chamber and aqueous (produced by the ciliary body), the vitreous, and the layered retina with the fovea as the point of highest acuity. The uvea comprises iris, ciliary body, and choroid.
Refractive conditions
Emmetropia, myopia (axial elongation versus refractive/curvatural or index causes), hyperopia, astigmatism, presbyopia (a loss of accommodative amplitude, not a clarity problem), aphakia, pseudophakia, anisometropia, and amblyopia. Presbyopia becomes clinically apparent around age 40, with adds starting near +1.00 D and progressing toward +2.50 D.
Traps
- Astigmatism terminology: with-the-rule astigmatism has the steepest corneal meridian vertical (near 90°). Name the steep meridian first, then remember the correcting minus-cylinder axis lies 90° away from it.
- Axial length: each millimeter of elongation adds roughly 2.5–3.00 D of myopia.
- Accommodation is measured in diopters and is the inverse of the near point in meters.
Worked rule of thumb
A cornea with anterior radius 7.8 mm and n = 1.376 has front-surface power F = (1.376 − 1) / 0.0078 m ≈ +48 D; the posterior surface subtracts about 6 D, leaving ~42–43 D for the cornea as a whole.