How much a higher index saves
A −5.00 D lens in a 52□18 frame for a 64 mm PD, the values the calculator opens with, in five materials:
| Index | Edge | Centre | Weight per lens |
|---|---|---|---|
| 1.50 | 5.7 mm | 1.5 mm | 11.4 grams |
| 1.56 | 5.3 mm | 1.5 mm | 10.7 grams |
| 1.60 | 5.0 mm | 1.5 mm | 10.3 grams |
| 1.67 | 4.6 mm | 1.5 mm | 10.9 grams |
| 1.74 | 4.3 mm | 1.5 mm | 10.4 grams |
From 1.50 to 1.67 the edge loses 1.1 mm. The weight hardly moves, and even rises at 1.67, because the calculator assumes a denser material above index 1.60 (see the limits below).
Frame size matters more than index
The same −5.00 D in 1.67: a 48□18 frame for a 64 mm PD gives a 3.8 mm edge, while a 56□18 frame for a 60 mm PD gives 6.1 mm. The lens radius is squared in the formula, so a smaller, well-centred frame often saves more than the next index up.
For plus lenses the saving is at the centre: +4.00 D in a 50□18 frame for a 64 mm PD comes out at 4.4 mm in 1.50 and 3.9 mm in 1.60.
How the estimate is made
c = (A + DBL − PD) ÷ 2
y = A ÷ 2 + c
s = y² × |F| ÷ (2000 × (n − 1))
- c
- Decentration per eye in mm, taken as 0 if the frame is narrower than the PD.
- y
- The radius of lens needed, in mm: half the eye size plus the decentration.
- s
- Sag, the thickness difference between centre and edge, in mm.
- F, n
- Lens power in dioptres and refractive index.
- 1.5 mm
- Minus lens: centre 1.5 mm, edge 1.5 + s. Plus lens: edge 1.5 mm, centre 1.5 + s.
Checked by hand for the starting values: c = (52 + 18 − 64) ÷ 2 = 3 mm, y = 29 mm, s = 29² × 5 ÷ (2000 × 0.67) = 3.14 mm, so the edge is 1.5 + 3.14 = 4.6 mm.
Limits
- Spherical power only. For a cylinder, enter the power in the meridian you are interested in; the oblique meridian calculator gives it.
- The lens is treated as round with radius y. The thickest point of a rectangular shape, and the real weight, will differ.
- The 1.5 mm minimum is a fixed assumption. Rimless and drilled mounts, grooves and safety lenses need more.
- Weight uses 1.2 g/cm³ up to index 1.60 and 1.35 g/cm³ above. Treat it as a comparison, not a specification.
- Aspheric designs are thinner than this spherical estimate. Confirm real figures with your lens supplier; see the medical disclaimer.