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Spectacle Prescription
Optics & Refraction
Teaching images
71. Introduction & Clinical Analogy
Clinical Analogy
- Think of the eye as a camera. The cornea and crystalline lens are the camera lens system, and the retina is the film. A spectacle lens corrects the optical error of this camera so the image falls precisely on the film. Prescribing spectacles is simply choosing the right external optical element to compensate for the eye's refractive error - the power it has vs. the power it needs.
- Spectacle prescribing is a core clinical skill integrating optics, refraction, patient physiology and practical prescribing logic. Errors in prescribing are a common cause of patient dissatisfaction and visual morbidity. A thorough PG-level understanding is essential for both exam success and clinical practice.
2. Prescription Notation & Format
Standard Notation
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1- A spectacle prescription is written as: Sphere / Cylinder × Axis with additions for near (near addition (ADD)) and prism if needed.
- Each element has clinical significance.
| Component | Symbol | Unit | Clinical Meaning |
|---|---|---|---|
| Sphere (SPH) | Sphere (S) / Sphere (Sph) | Dioptres (D) | • Corrects simple myopia (–) or hyperopia (+). • Negative = concave. • Positive = convex. |
| Cylinder (CYL) | Cylinder (C) / Cylinder (Cyl) | Dioptres (D) | • Corrects astigmatism. • Power in the meridian perpendicular to the axis. |
| Axis | Axis (Ax) / × | Degrees (1–180°) | • Meridian of NO cylinder power. • Always specified with cylinder. |
| Addition (ADD) | Addition (Add) | Dioptres (D) | • Extra positive power for near vision in presbyopia. |
| Prism | ∆ | Prism dioptres (∆) | • Corrects binocular imbalance / diplopia. |
| Base Direction | Base out (BO) / Base in (BI) / Base up (BU) / Base down (BD) | - | • Direction of prism base: Out, In, Up, Down. |
| Back vertex distance (BVD) | Millimetres (mm) | • Distance from back surface of lens to cornea. • Important in high-powered lenses; compensate if a spectacle lens has >7.00 D in either principal meridian and fitting vertex distance differs from refracting vertex distance. |
| Component | Symbol | Unit | Clinical Meaning |
|---|---|---|---|
| Sphere (SPH) | Sphere (S) / Sphere (Sph) | Dioptres (D) | • Corrects simple myopia (–) or hyperopia (+). • Negative = concave. • Positive = convex. |
| Cylinder (CYL) | Cylinder (C) / Cylinder (Cyl) | Dioptres (D) | • Corrects astigmatism. • Power in the meridian perpendicular to the axis. |
| Axis | Axis (Ax) / × | Degrees (1–180°) | • Meridian of NO cylinder power. • Always specified with cylinder. |
| Addition (ADD) | Addition (Add) | Dioptres (D) | • Extra positive power for near vision in presbyopia. |
| Prism | ∆ | Prism dioptres (∆) | • Corrects binocular imbalance / diplopia. |
| Base Direction | Base out (BO) / Base in (BI) / Base up (BU) / Base down (BD) | - | • Direction of prism base: Out, In, Up, Down. |
| Back vertex distance (BVD) | Millimetres (mm) | • Distance from back surface of lens to cornea. • Important in high-powered lenses; compensate if a spectacle lens has >7.00 D in either principal meridian and fitting vertex distance differs from refracting vertex distance. |
Exam pearl
- Axis is the meridian of ZERO cylinder power, not maximum. The full cylinder power acts in the meridian 90° away from the axis.
Exam pearl
- Prescription shows −2.00 / −1.00 × 180.
- The cylinder power is −1.00 dioptres acting along the 90° meridian
- (perpendicular to axis 180°). Many candidates confuse 'axis' with 'meridian of action'. In exams, always state:
- 'cylinder acts at 90° to its axis.'
Transposition of Prescriptions
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1- Converting a minus-cylinder prescription to plus-cylinder form (or vice versa) is called transposition.
- Optometrists use minus cylinder; ophthalmologists traditionally use plus cylinder in many countries.
| Step | Rule | Example |
|---|---|---|
| 1. Change sphere | • New Sphere (SPH) = Old Sphere (SPH) + Old Cylinder (CYL) | • −3.00 + (−1.00) = −4.00 |
| 2. Change cylinder sign | • New Cylinder (CYL) = same magnitude, opposite sign | • −1.00 → +1.00 |
| 3. Change axis | • New Axis = Old Axis ± 90° | • 180° → 90° |
| Result | • Equivalent prescription | • −3.00/−1.00×180 ≡ −4.00/+1.00×90 |
| Step | Rule | Example |
|---|---|---|
| 1. Change sphere | • New Sphere (SPH) = Old Sphere (SPH) + Old Cylinder (CYL) | • −3.00 + (−1.00) = −4.00 |
| 2. Change cylinder sign | • New Cylinder (CYL) = same magnitude, opposite sign | • −1.00 → +1.00 |
| 3. Change axis | • New Axis = Old Axis ± 90° | • 180° → 90° |
| Result | • Equivalent prescription | • −3.00/−1.00×180 ≡ −4.00/+1.00×90 |
Exam pearl
- Key Concept: Transposition is the single most tested optical calculation in PG exams. Memorise the 3-step rule. Verify: the spherical equivalent (SE) must remain identical in both forms. SE = Sphere + ½ Cylinder.
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Keep reading Spectacle Prescription
9 more sections are included with the Clinical plan.
- Refractive Errors - Pathophysiology & Correction
- Principles of Spectacle Prescribing
- Special Prescribing Situations
- Lens Design, Materials & Coatings
- Troubleshooting Spectacle Intolerance
- Evolving Concepts & Landmark Evidence