Prisms in Spectacle Lenses

When your eye exam detects a misalignment, a prismatic correction may be built into your lenses. Prisms help the eyes properly align by shifting the image so both eyes see it in a comfortable position. This doesn’t change vision clarity (sharpness), but it does reduce the strain required to fuse images.

Why prisms are prescribed

Prism in spectacles is most commonly used to help with:

Double Vision (Diplopia)

Diplopia occurs when the brain receives two separate images instead of one unified image. Prism bends light so the images align properly, allowing comfortable, single binocular vision.

Positional Misalignment

Even if both eyes see clearly, they might see the image slightly above, below, left, or right of one another. Prisms shift images so the eyes are at rest rather than under continuous strain.

Convergence Issues

When focusing on near objects (like reading), the eyes must converge. Some people struggle to do this efficiently, causing words to move, blur, or feel like they’re swimming together. Prisms reduce the effort needed to maintain a single image during reading.

Visual Therapy

Visual therapy is prescribed to improve the brain’s ability to coordinate and process visual information. Through targeted exercises and activities, it enhances eye teaming, focus flexibility, and binocular integration, helping patients achieve more comfortable and efficient visual function.

Why Eye Alignment Matters

To see comfortably and effectively, both eyes must work together as a team. When the eyes are slightly misaligned, the extra effort to fuse images places strain on the eye muscles. This can lead to symptoms like eye fatigue, headaches, or occasional double vision, especially when tired.

Sometimes this misalignment isn’t obvious, but even small discrepancies as little as a fraction of a millimetre can cause significant visual stress.

Signs You Might Benefit from Prismatic Correction

In children, convergence problems are often treated with eye exercises rather than prism, but prism may still be needed in certain cases.

Adapting to Prismatic Lenses

It’s normal to experience mild discomfort, slight headaches, or unusual sensations when you first wear lenses with prism. This is part of your visual system adapting to a more relaxed alignment.

Most people report that symptoms reduce or disappear within a few days as the eyes adjust.

Lens Materials and Refractive Index (Why it Matters)

Prismatic lenses can be made in the same materials used for regular spectacle lenses. These materials influence weight, thickness, durability, and how much light bends.

Common materials:

Refractive Index Explained

The refractive index tells us how much a material bends light. Higher index materials bend light more, allowing thinner lenses but also reflecting more light. For example:

• - CR-39: ~1.498
• - High-index plastic: up to ~1.74

Because higher index lenses reflect more light, it’s common to add an anti-reflective coating to improve comfort and clarity.

Clinical References

1. Hoya Specialty Lenses (n.d.) Prism in eyeglass lenses and specialty optics. Available at: https://www.hoyavision.com (Accessed: 23 January 2026).

2. GlassesCrafter (n.d.) What is prismatic correction in glasses?. Available at: https://www.glassescrafter.com (Accessed: 23 January 2026).

3. Wikipedia (n.d.) Prism (optics). Available at: https://en.wikipedia.org/wiki/Prism_(optics) (Accessed: 23 January 2026).

4. Eye Doctor Q&A – Eyeglasses and Eyeglass Lenses (n.d.) Prism and eye alignment topics. Available at: https://www.eyedoctorqa.com (Accessed: 23 January 2026).