Prism is a specialized lens correction for eye alignment problems — it does not treat nearsightedness, farsightedness, or astigmatism.
How prism lenses work
Prism lenses shift the image entering your eye so the images seen by each eye align properly. This eliminates double vision, reduces eye strain from constant alignment effort, and helps your eyes work together as a team.
How it appears on your prescription
A prism prescription has two key values:
- Prism power: Measured in prism diopters (Δ), the strength of the correction.
- Base direction: The orientation of the prism, marked as:
- Base In or Base Out (for horizontal alignment)
- Base Up or Base Down (for vertical alignment)
Prism only appears on prescriptions for people who need it — it is not a routine part of most glasses prescriptions.
When prism is prescribed
Prism is most commonly used for:
- Double vision (diplopia)
- Strabismus (visibly misaligned eyes)
- Hidden eye misalignment (phoria) that causes frequent eye strain or headaches
- Certain binocular vision dysfunctions that make close work tiring
Note
Prism correction requires precise measurement. A short adjustment period is normal for first-time wearers. If discomfort lasts more than 1–2 weeks, follow up with your eye doctor.
