Ophthalmic Lenses

Single Vision Lenses

- Have one prescription power throughout the entire lens.

- Used to correct myopia, hyperopia, or astigmatism.

Multifocal Lenses

Bifocals — Two distinct viewing zones: distance on top, near (segment/seg) on bottom.

- Common styles: Flat-Top (D-seg) 28mm, Flat-Top 35mm, Round seg, Executive (Franklin)

- The Executive bifocal has a seg that extends the full width of the lens.

Trifocals — Three zones: distance (top), intermediate (middle), near (bottom).

- The intermediate zone is typically half the add power.

Progressive Addition Lenses (PALs) — No visible lines; a smooth transition from distance through intermediate to near.

- Advantages: cosmetically appealing, no image jump, continuous vision at all distances.

- Disadvantages: peripheral distortion/swim, adaptation period, narrower intermediate and near zones.

- The fitting cross is positioned at the center of the pupil.

Lens Forms (Meniscus)

- Most ophthalmic lenses are meniscus shaped — convex on the front, concave on the back.

- Plus lenses — Thicker in the center, thinner at the edges.

- Minus lenses — Thinner in the center, thicker at the edges.

CR-39 (Columbia Resin #39) — Index: 1.50, Abbe: 58. The standard plastic lens material. Lightweight, good optical clarity, accepts tints well.

Polycarbonate — Index: 1.586, Abbe: 30. High impact resistance — required for children and safety eyewear. Built-in UV protection. Requires scratch-resistant coating.

Trivex — Index: 1.53, Abbe: 45. Excellent impact resistance, lightest lens material, superior optical clarity. Built-in UV protection.

High-Index Plastics (1.60, 1.67, 1.74) — Thinner and lighter for strong prescriptions. Higher index = thinner lens but lower Abbe value. Recommended for prescriptions above ±4.00 D.

Crown Glass — Index: 1.523, Abbe: 59. Excellent optical clarity, scratch-resistant. Heavy and prone to shattering — rarely used today.

Anti-Reflective (AR) Coating — Reduces reflections, improves clarity. Applied to both surfaces.

Scratch-Resistant Coating — Required for polycarbonate and high-index lenses.

UV Protection — Polycarbonate and Trivex have built-in 100% UV protection. CR-39 requires a UV coating.

Photochromic Lenses — Darken in UV light, return to clear indoors. May not fully darken in a car.

Polarized Lenses — Block horizontally oriented reflected glare. Excellent for driving, fishing, and outdoor activities.

Blue Light Filtering — Reduces exposure to high-energy visible (HEV) blue light from digital screens.

Base Curve — The front (convex) curve of the lens. Choosing the correct base curve minimizes peripheral aberrations.

Nominal Lens Formula: Front Surface Power + Back Surface Power = Total Power

Focal Length: f (in meters) = 1 / D (diopter power)

Transposition — Converting between plus and minus cylinder forms:

1. Add the sphere and cylinder together → new sphere.

2. Change the sign of the cylinder.

3. Change the axis by 90 degrees.

Optical Center — The point on the lens where light passes through without deviation. Should align with the patient's pupil center.

Vertex Distance — The distance between the back surface of the lens and the cornea (typically 12-14mm). Compensation needed for prescriptions over ±4.00 D.