Eye spherical, in the context of vision correction, refers to the nature of the lens power needed to correct nearsightedness or farsightedness. It means that the correction is equal in all meridians of the eye. This is represented by the "Sphere" (SPH) value in an eyeglass prescription.
Understanding Spherical Correction
When your eye has a spherical refractive error, it means the light focuses either too far in front of or too far behind the retina, and this focusing error is uniform across all angles of the eye. This differs from astigmatism, where the eye has different focusing powers at different angles.
Key Points:
- Uniform Correction: Spherical correction means the same lens power is needed regardless of the orientation of the eye.
- Nearsightedness (Myopia): This occurs when light focuses in front of the retina. A negative (-) sphere value is used to correct this, as it's a concave lens to diverge light.
- Farsightedness (Hyperopia): This occurs when light focuses behind the retina. A positive (+) sphere value is used to correct this, using a convex lens to converge light.
- Diopters (D): Lens power is measured in diopters. The higher the absolute value (positive or negative), the stronger the lens power needed.
Spherical Correction in Practice
| Condition | Light Focus | Correction Type | Sphere Value |
|---|---|---|---|
| Nearsightedness | In front of retina | Concave lens | Negative (-) |
| Farsightedness | Behind the retina | Convex lens | Positive (+) |
For example:
- A prescription of -2.00 SPH indicates nearsightedness needing a 2.00 diopter concave lens.
- A prescription of +1.50 SPH indicates farsightedness needing a 1.50 diopter convex lens.
In essence, eye spherical refers to the lens power uniformly needed to bring light into sharp focus on the retina, correcting for basic nearsightedness or farsightedness. It deals with the overall focusing power required rather than variations in that power across different parts of the eye.
