Lens picker · High index
Do you need high index lenses?
High index means the lens material bends light more, so the same correction can be ground in less material. It changes the shape of the lens, never the correction. Whether the change is worth the money depends on one number on your paper and one thing about your frame.
What index is
Refractive index is the ratio between the speed of light in air and in the material. Standard plastic sits at 1.50. Push it to 1.74 and light bends far enough in a thinner piece to do the same job, so a minus lens loses much of the thick rim that would otherwise sit at its edge.
The correction on the paper does not change. A −5.00 lens is −5.00 at every index; what changes is how much material stands between the frame and the edge of the optical zone.
The thickness against Abbe trade
Every step of index costs clarity. The Abbe number falls as the index rises — CR-39 sits at 58, the 1.60 materials near 42, and 1.67 and 1.74 at 32 and 33. Low Abbe shows as a coloured fringe on high-contrast edges seen through the outer part of the lens, most noticeably at night against headlights.
High-index materials also reflect more light at the surface, which is why anti-reflective coating stops being an upsell at 1.67 and becomes part of how the lens is sold.
The bands
The trade works to four steps, set by the strongest sphere on the paper: up to 2.00 D standard 1.50, from 2.00 to 4.00 D the 1.59 or 1.61 step, from 4.00 to 6.00 D 1.67, past 6.00 D 1.74. Those are the same bands the calculator below uses.
Between 1.67 and 1.74 the difference in finished edge is under a millimetre at most powers, which is the step people most often pay for and least often notice.
When 1.50 is fine
Below about 2.00 D the edge on a standard lens is a couple of millimetres in a normal frame, and the bevel hides most of it. At that power the argument for a thinner material is weight rather than looks, and CR-39 is the clearer material anyway.
Frame size
The edge of a minus lens gets thicker the further you go from its centre, so the finished thickness depends on how wide a disc the frame asks the shop to cut. A small eye size hides roughly a third of the edge a medium frame would show; a large one adds about a quarter to it. Those two figures are approximations of a shop's edging, not a lab calculation.
At a strong number, dropping one frame size does more for the finished edge than one step of index does, and costs nothing.
Common questions
Are high index lenses worth it?
Below about 3.00 D the difference is a millimetre or two of edge that most frames hide. From 4.00 D on it changes how the glasses sit and weigh. The cost is colour fringing: Abbe 32 and 33 at 1.67 and 1.74, against 58 for CR-39.
What is the difference between 1.67 and 1.74 lenses?
1.74 is the thinnest glazing sold and 1.67 is close behind at lower cost and in more coating options. At most powers the finished edge differs by under a millimetre, so 1.74 is usually ordered for very strong numbers or rimless mounts.
Do high index lenses distort vision?
They do not change the correction. What people report is colour fringing on high-contrast edges seen through the outer part of the lens, which follows from the lower Abbe number of the material, and more surface reflection unless anti-reflective coating is glazed on.
What index do I need for −4.00?
−4.00 D sits at the top of the 1.59/1.61 band and at the start of the 1.67 one, so both are ordered at that number. Frame size decides it: a small frame keeps 1.61 sensible, a large one is where 1.67 starts to show.
Run it on your own numbers
Put the sphere numbers from your own paper in below and the picker draws the edge at 1.50 against the index your numbers land on, at your frame size.
Prescription
Copy the numbers exactly as printed. Blank boxes are fine.
| SPH | CYL | AXIS | ADD | |
|---|---|---|---|---|
| OD | ||||
| OS |
Mostly for
Frame
Age band
The SPH boxes are where this page starts: the strongest one sets the index band.
Nothing leaves your phone. The paper is only used on this page.
The rest of the picker
EndMyopia treats myopia as a refractive state, not a medical condition. This is a free educational tool — not medical advice, diagnosis, or a substitute for a licensed optometrist. Your optician sets your correction and the shop grinds the lenses.
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