Diameter Is the Free Lever: How Frame Size Sets the Weight and Edge of a Minus Lens

Minus lenses get heavy at their edges, and the edge is bought by diameter. This paper computes sagitta, edge thickness, and mass for minus lenses of -2, -4, and -6 D cut to diameters from 50 to 70 mm, from stated surface geometry for a refractive index of 1.50 and a 1.0 mm centre. A -6 D lens at 70 mm has an edge 8.71 mm thick and a mass of 24.7 g; the same lens cut to 55 mm has an edge 5.67 mm thick and a mass of 10.5 g, a saving of 57.5 percent, while the equivalent saving at -2 D is only 51.3 percent. The frame-selection rule follows: for minus powers of 4 D and above, choose the smallest frame whose lens shape still covers the pupil's excursions, because diameter, not material marketing, is the largest honest lever on weight and edge thickness. This study is simulated, not measured.