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Spectacle prescribing
Anatomy & Optics
Key Points
- •Vertex distance matters for Rx >±4 D: minus lens closer → less effective; plus lens closer → more effective. Formula: Fe = F / (1 − d.F)
- •Knapp's rule: spectacle correction causes minimal aniseikonia in axial anisometropia; contact lenses are preferred for refractive anisometropia
- •Aniseikonia becomes symptomatic when image size difference exceeds 3–5% (~2% per diopter of spectacle power difference); contact lenses reduce aniseikonia
- •Bifocal segment height: at lower pupillary margin for children with accommodative esotropia (executive type with +3.00 add); at mid-pupil for most adults
- •Slab-off prism corrects vertical prismatic imbalance (Prentice rule) in the reading position of anisometropic spectacles - applied to the more minus lens
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- Definition
- Epidemiology
- Aetiology and causes
- Pathogenesis
- Classification
- Risk factors and associations