26.2 Vision Correction

Vision Defects and Correction

Common Vision Defects

A healthy eye focuses light directly onto the retina, producing a sharp image. Vision defects occur when the eye's shape or the cornea's curvature causes light to focus at the wrong point. Here are the main types:

• Myopia (nearsightedness) — Distant objects appear blurry while close objects stay clear. The eyeball is too long or the cornea too curved, so light converges in front of the retina instead of on it.
• Hyperopia (farsightedness) — Close objects appear blurry while distant objects stay clear. The eyeball is too short or the cornea too flat, so light would converge behind the retina.
• Astigmatism — Vision is blurred or distorted at all distances. The cornea or lens is irregularly shaped (more like a football than a basketball), causing light to focus at multiple points rather than one.
• Presbyopia — An age-related loss of the lens's flexibility. The lens can no longer change shape enough to focus on nearby objects, which is why many people need reading glasses starting in their 40s.

Corrective Lenses

Corrective lenses fix refractive errors by bending light before it enters the eye, shifting the focal point onto the retina.

• Myopia is corrected with a concave (diverging) lens. This lens spreads light rays outward, pushing the focal point farther back so it lands on the retina.
• Hyperopia is corrected with a convex (converging) lens. This lens bends light rays inward, pulling the focal point forward onto the retina.

Lens strength is measured in diopters (D), defined as the inverse of the focal length in meters:

$P = \frac{1}{f}$

where $P$ is the power in diopters and $f$ is the focal length in meters. A converging lens has a positive power (e.g., +1.5 D for hyperopia), while a diverging lens has a negative power (e.g., -2.25 D for myopia). A higher absolute value means a stronger lens and a shorter focal length.

Laser Vision Correction (LASIK)

LASIK (laser-assisted in situ keratomileusis) corrects refractive errors by permanently reshaping the cornea with an excimer laser. The procedure follows these steps:

1. A thin flap is created in the outer layer of the cornea.

2. The flap is folded back to expose the underlying corneal tissue.

3. The excimer laser removes microscopic amounts of tissue to reshape the cornea:

   • For myopia: the cornea is flattened, reducing its focusing power.
   • For hyperopia: the cornea is steepened, increasing its focusing power.

4. The corneal flap is repositioned and heals naturally without stitches.

Vision typically improves within a few days. The main advantages are reduced or eliminated need for glasses/contacts, quick recovery, and a high success rate. Risks include dry eyes, glare, halos around lights, and the fact that age-related changes (like presbyopia) can still develop later.

Eye Care Professionals and Vision Assessment

• Ophthalmologists are medical doctors who specialize in eye care, including surgery and treatment of eye diseases.
• Optometrists are primary eye care providers who diagnose vision problems and prescribe corrective lenses but typically do not perform surgery.

Visual acuity is measured using a Snellen chart. A score of 20/20 means you can read at 20 feet what a person with normal vision reads at 20 feet. A score of 20/40 means you need to be at 20 feet to read what a normal eye reads at 40 feet.

Accommodation is the eye's ability to shift focus between distant and near objects by changing the shape of the lens. The ciliary muscles contract to thicken the lens for close-up focus and relax to flatten it for distance. This process relies on the refractive properties of the eye's components (cornea, lens, aqueous humor, and vitreous humor), each of which bends light as it passes through.