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Method

How it works

Plain words first. The formulas are at the bottom for anyone who wants to check our working.

Not yet reviewed by a licensed optician · Updated [date] · Engine v1.0.0

The short version

Lenses work by being curved. A minus lens (for short sight) is thin in the middle and thick at the edge. A plus lens (for long sight) is the opposite.
Distance from the centre is what makes edges thick. Thickness grows with the square of that distance, so a few extra millimetres of lens width add a lot.
Your eyes don't move to fit the frame. If the frame is wider than your eyes are apart, each lens centre is shifted toward your nose (opticians call this decentration). That pushes the outer edge even further out.
Higher index materials bend light more, so they need less curve for the same strength. That's why 1.67 and 1.74 are thinner, but each step saves less than the one before.

Lenses work by being curved. Minus lenses are thick at the edge; plus lenses in the middle.
Distance from the centre makes edges thick, and it counts squared.
A frame wider than your eyes shifts each lens centre toward your nose, pushing the outer edge further out.
Higher index needs less curve, but each step saves less.

Why frame fit can beat a pricier lens

+ marks the middle of each lens shape. Blue dots mark your pupils, where each lens centre must sit.

4 mm shift each side

In this 52 □ 18 frame, the outer edge of each lens ends up 30 mm from its optical centre. Pick a 48 mm lens width and that becomes 26 mm. Because thickness grows with the square of that distance, the edge at −6.00 in 1.67 drops from about 5.5 mm to 4.4 mm. Upgrading the same 52 mm frame to 1.74 only gets you to 5.1 mm.

In a 52 □ 18 frame at −6.00, 1.67 is about 5.5 mm. A 48 mm frame brings it to 4.4 mm; 1.74 in the wide frame only reaches 5.1 mm.

What we calculate

Edge thickness all the way round the frame shape, at 72 points, for each eye and each material.
Centre thickness, which matters most for plus lenses.
The thickest point and where it sits: outer side, nose side, top or bottom.
Weight per pair, from the lens volume inside your frame shape and each material's density.
What each change saves: narrower frame, better PD match, aspheric design, next material up, and rim type.

What we don't do yet

Prism and reading additions (ADD). Varifocals and prism can add thickness. Enter your distance numbers and ask your optician.
Wrap-around frames with strong curve. Treat results as a rough guide.
Each lab's own lens blanks. Labs choose surfaces and minimum thicknesses slightly differently. That's the main reason real lenses vary by a few tenths of a millimetre.

For experts: the maths

Surfaces and thickness

Sag of a spherical surface of power F (dioptres) at distance y (mm) from the optical centre, material index n:

R = 1000 · (n − 1) / F

s(y) = R − √(R² − y²)

Minus lens: edge thickness in direction θ, centre thickness t_c set by the material minimum:

t_e(θ) = t_c + s_back(r(θ)) − s_front(r(θ))

Plus lens: centre thickness set so the thinnest edge meets the rim minimum t_e,min:

t_c = t_e,min + s_front(r_max) − s_back(r_max)

Cylinder: power in meridian θ (minus-cylinder form after conversion):

F(θ) = S + C · sin²(θ − axis)

Decentration per eye, with binocular PD or monocular PD_eye:

d = (A + DBL − PD) / 2

or d = (A + DBL) / 2 − PD_eye

r(θ) is the distance from the shifted optical centre to the frame outline (shape template scaled to A × B). Aspheric designs reduce back-surface sag by a design factor k = 0.88 (placeholder, all materials).

Defaults we use

Per-material defaults. Densities are typical published values.
Material	Index n	Density g/cm³	Min centre, minus
Standard (CR-39)	1.50	1.32	2.0 mm
Polycarbonate	1.59	1.20	1.0 mm
Mid index	1.60	1.30	1.2 mm
High index	1.67	1.35	1.1 mm
Highest index	1.74	1.47	1.1 mm

Swipe the table sideways for more columns.

Minimum edge for plus lenses: full rim 1.0 mm, semi-rimless 1.8 mm, rimless 2.0 mm. Front curve chosen from a standard base-curve chart by power: [chart reference].

Weight and the recommendation

m = ρ · ∬ t(x, y) dA   (over the frame outline, both lenses)

Recommended material: the cheapest one whose thickest point is within 0.5 mm of the thinnest available, chosen per eye. Prices never change which material is the thinnest; they only break ties.

How accurate is it?

We measure real lenses and publish every one next to our prediction, misses included.

See measured lenses

Changelog

Engine changes by version
Version	Date	Change
v1.0.0	[date]	First public version.

Estimates for comparison. Your optician or lab will confirm final thickness.