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Spectacle Magnification
Power & Shape Factors
Calculate the magnification or minification effect of spectacle lenses. Important for aniseikonia assessment.
Required Inputs
Optional: Shape Factor Inputs(for complete analysis)
Understanding Spectacle Magnification
Power Factor = 1 / (1 - d x F)
Shape Factor = 1 / (1 - (t/n) x F1)
SM = Power Factor x Shape Factor
d = vertex distance (m), F = lens power (D), t = center thickness (m), n = refractive index, F1 = front curve (D)
Key Concepts:
- •Power factor depends on vertex distance and lens power
- •Shape factor depends on lens thickness and front curve
- •1.0x = no change in image size
- •>1.0x = magnification (plus lenses)
- •<1.0x = minification (minus lenses)
Aniseikonia Considerations
Aniseikonia is a difference in perceived image size between the two eyes. It commonly occurs with significant Rx differences between eyes (anisometropia).
Tolerable Difference
Up to 2-3% difference is usually tolerated
Problematic
>5% may cause diplopia, headaches, depth perception issues
Solutions for Aniseikonia:
- •Contact lenses (minimal magnification)
- •Iseikonic lens design (modify thickness/curves)
- •Reduce vertex distance for minus lenses
- •Match base curves between eyes when possible