Bifocal Lenses
As the name suggests, bifocal lenses combine two optical powers into a single lens. They appear similar to a single vision lens but include a visible near segment in the lower portion of the lens. This segment is typically set for a fixed near working distance, most commonly 40 cm for reading.
Bifocals are designed for people who need clear distance vision as well as consistent near vision, without the gradual power changes found in progressive lenses. The size, position, and power of the near segment can be customised to suit occupational and lifestyle needs.
Types of Bifocal Lenses
Different bifocal designs mainly differ in segment shape, size, and ease of adaptation:
- - Flat-Top (D-Style) Bifocals: The most commonly prescribed bifocal. The near segment resembles a sideways “D” with a flat upper edge, creating a clear transition between distance and near vision.
- - Kryptok (Round Segment) Bifocals: The near portion is a full circular segment, usually 25 mm in diameter. The transition is less distinct, but the round shape can slightly limit the upper field of view.
- - Curved-Top (C-Style) Bifocals: Similar to flat-top lenses, but with a gently curved upper edge on the segment, providing smoother peripheral vision.
- - Franklin / Executive Bifocals: The near portion spans the full width of the lens, offering the widest possible reading area for extensive near work.
Lens Materials
Bifocal lenses are available in several materials, each with different optical and safety properties:
- - Glass: Traditionally made using crown glass. While optically excellent, glass lenses are heavier and less impact resistant.
- - CR-39 Plastic: Lightweight, cost-effective, and optically clear. This remains one of the most commonly used materials.
- - Polycarbonate: Highly impact resistant and lighter than CR-39. Ideal for safety eyewear and children’s spectacles.
- - Trivex: Offers similar impact resistance to polycarbonate but with improved optical clarity and lower weight.
Refractive Index Explained
The refractive index describes how efficiently a lens material bends light. Higher-index materials bend light more efficiently, allowing lenses to be made thinner for the same prescription.
- - CR-39 plastic: ~1.498
- - Polycarbonate: ~1.61
- - High-index plastics: up to 1.74
- - Glass: 1.53–1.9
Note: Higher-index lenses tend to reflect more light, which is why anti-reflective (AR) coatings are strongly recommended.
Tints and Protective Coatings
Bifocal lenses can be customised with a wide range of enhancements to suit your visual environment:
Tints & UV Protection
Available as solid or gradient tints. Light tints are ideal for indoor/office use, while UV protection is essential to reduce long-term eye damage.
Anti-reflective (AR) Coating
Reduces glare from headlights and screens, improving clarity and overall cosmetic appearance.
Durability & Blue Light
Scratch-resistant coatings improve longevity, while optional blue-light filtering helps during heavy digital device use.
Regular eye exams are crucial. Vision lost to undiagnosed conditions cannot be restored; check-ups save sight.
Clinical References
1. World Optic – Bifocal Lenses. Available at: https://worldoptic.com/bifocal-lenses
2. All About Vision – Types of Bifocal Lenses. Available at: https://www.allaboutvision.com/eyeglasses/bifocals/
3. Essilor – Understanding Bifocal Lenses. Available at: https://www.essilor.com/en/vision-care/bifocal-lenses/