Visual Optics and Refraction
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Direct answer
The eye behaves as a compound optical system with a total refractive power of about 60 dioptres when unaccommodated, of which the cornea contributes roughly 43 D and the crystalline lens only about 15-17 D. Because the cornea separates air from aqueous, its smaller refractive index gradient still dominates the eye's power. In emmetropia the parallel rays from a distant object come to a focus exactly on the retina; in myopia they focus in front of it, in hypermetropia behind it, and in astigmatism different meridians focus at different points. Correction always moves the far point to infinity: concave lenses for myopia, convex for hypermetropia, cylindrical for astigmatism.
What you must remember
- Power shares: cornea about 43 D (nearly two-thirds), lens 15-17 D; total unaccommodated power about 60 D, axial length about 24 mm.
- Listing's reduced eye: a single refracting surface of +60 D with the principal point about 2 mm behind the anterior corneal surface and the nodal point about 5.6 mm behind it; the second focal point falls on the retina in emmetropia.
- Far point and near point: in emmetropia the far point is at infinity and the near point at about 25 cm, the distance used for conventional near work.
- Myopia: axial (longer globe, the common form) or refractive (too much power); corrected with a concave (minus) lens; the far point is finite, and its distance in metres equals 1 divided by the dioptres of error.
- Hypermetropia: rays focus behind the retina; corrected with a convex (plus) lens; young patients partly compensate by accommodation, so latent hypermetropia is unmasked under cycloplegia.
- Astigmatism: different curvature in different meridians, usually corneal; corrected with a cylindrical lens, often combined spherocylindrically.
- Presbyopia: not a refractive error but loss of accommodative amplitude with age; reading addition (plus power) is the treatment.
- Refractive surgery: LASIK reshapes the cornea to flatten it in myopia; it acts on the cornea, which carries most of the eye's power.
How to work through a refractive error
Start by asking where parallel rays focus. Take a 20-year-old who cannot read the blackboard but reads small print easily: the myopic blur at distance means the image falls in front of the retina, so the far point is finite. Retinoscopy shows, say, −2 D; his far point is 0.5 m, which is why he sees his phone clearly at arm's-length-closer distances without glasses. The correcting lens must make distant objects appear to come from the far point: a −2 D concave lens does exactly that.
Now take a 45-year-old with asthenopia after near work and slightly blurred distance vision. Uncorrected visual acuity is 6/9, improving with a +1.5 D sphere, but under atropine or homatropine cycloplegia the refraction reads +2.5 D — the extra dioptre was latent hypermetropia being masked by accommodation. Prescribing the full cycloplegic value, or gradually building up to it, relieves the ciliary spasm. Finally, a patient who sees vertical lines sharply but horizontal ones blurred has astigmatism: the cornea is steeper in one meridian, and only a cylindrical lens, axis set against the steep meridian, equalises the focal planes.
Where students slip
Two slips recur in vivas. First, candidates say the lens is the main refracting element because it is "the focusing structure"; it is not — the air-cornea interface carries about two-thirds of the power, which is precisely why corneal laser surgery works and why vision blurs under water, where the air-cornea interface is lost. Second, the far point in myopia is confused with the near point. Remember the rule that ties them together: the far point of a myope equals minus one over the refractive error in dioptres, so a −5 D myope has a far point of 20 cm and can read small print unaided — a classic clue in clinical case stems.
Frequently asked questions
Why does the cornea contribute more refractive power than the lens?
The cornea separates air (refractive index 1.00) from aqueous (about 1.33), the largest index step in the eye, whereas the lens is surrounded by media of nearly its own index. Power depends on this index difference, not on curvature alone.
What is Listing's reduced eye?
A simplified single-surface model of the eye with +60 D power, used to calculate image size and refraction; the retinal image height equals object height times its distance from the nodal point divided by the nodal-point-to-retina distance.
How is axial myopia different from refractive myopia?
Axial myopia has an elongated globe with normal corneal power and is the commoner form; refractive myopia has normal length but excessive corneal or lenticular power, as in keratoconus or nuclear sclerosis.
Why does a diver see blur under water?
Water (index about 1.33) replaces air at the corneal surface, abolishing the air-cornea refraction that supplies two-thirds of the eye's power; the eye becomes strongly hypermetropic, needing a curved mask faceplate to restore the interface.
Which lens corrects astigmatism and how is the axis chosen?
A cylindrical lens corrects the meridian of mismatched curvature; the axis is set along the meridian that needs no correction, so power is added only in the steep, out-of-focus meridian.