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Spectacle magnification
The power-factor approximation for how much a spectacle lens changes retinal image size: SM = 1 / (1 - d x F), with d the vertex distance in meters. Plus lenses magnify, minus lenses minify, and both effects grow as the lens sits farther from the eye. The complete formula adds a shape factor from the front curve, thickness, and index; the power factor alone is the common exam form.
Spectacle magnification (power factor)
The common exam approximation: only the power factor, with the shape factor (front curve, thickness, index) omitted. Plus lenses magnify, minus lenses minify, and both effects grow with vertex distance.
OD
SM = 1.1364, about 13.6 percent larger retinal image
SM = 1 / (1 - d x F) = 1 / (1 - 0.012 x +10.00)
OS
SM = 0.8929, about 10.7 percent smaller retinal image
SM = 1 / (1 - d x F) = 1 / (1 - 0.012 x -10.00)
Go deeper
The same 1 - d x F denominator drives vertex compensation, which is why high-power corrections change both effective power and image size when they move.
Frequently asked questions
How much does a spectacle lens magnify?
The power-factor approximation is SM = 1 / (1 - d x F), with d the vertex distance in meters. A +10.00 D lens at 12 mm magnifies about 13.6 percent; a -10.00 D lens minifies about 10.7 percent.
Why do high-minus wearers see smaller images?
A minus lens diverges light, shrinking the retinal image; the farther the lens sits from the eye the stronger the effect. That is one reason contact lenses, worn at the cornea, change image size far less.
Is this the full spectacle magnification formula?
No. The complete formula multiplies this power factor by a shape factor built from the front curve, thickness, and index. The power factor alone is the common exam approximation and is labeled as such here.