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Magnification in optics
Anatomy & Optics
Key Points
- •Angular magnification of a simple magnifier: M = P/4 (P = power in dioptres, based on 25 cm reference distance)
- •Direct ophthalmoscope provides 15× magnification with ~5° field; indirect ophthalmoscope with +20 D lens provides 3× magnification with ~8 disc diameter field - higher condensing lens power = lower magnification but wider field
- •Relative distance magnification is the simplest form - halving the viewing distance doubles the magnification
- •Spectacle magnification causes aniseikonia in anisometropia; contact lenses minimise this by eliminating vertex distance
- •Kestenbaum reciprocal method: required add (D) = 1 / decimal VA (e.g., VA 6/60 = 0.1 → needs +10 D add)
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