How Lenses Form Images

The job of a lens is to make light focus or diverge according to what we need.

Convex vs Concave Lenses

• Convex lens (converging): brings parallel rays together at a focal point; used to correct hyperopia or presbyopia, letting light focus earlier.
• Concave lens (diverging): spreads parallel rays apart first, then lets the eye’s own system focus them; used to correct myopia, pushing the focal point “back” onto the retina.
• The shorter the focal length, the more sharply light changes direction; the thicker the center of a convex lens, the stronger it converges; the thinner a concave lens, the stronger it diverges.

Common Lens Specifications

• Refractive index: the higher it is, the thinner the lens, but the Abbe number usually drops and dispersion increases slightly.
• Abbe number: a higher value means less dispersion and fewer visible color fringes.
• Coatings: anti-reflection (reduces glare), blue-light filtering, and hardening (scratch resistance).

In the next section, we will put the concept of a lens inside the “eye model” to see the imaging result.