A higher lens index lets a lens bend light with less curvature, so the same prescription can often be made thinner. But index is not a quality grade, and the highest number is not automatically the right choice. Your full prescription, frame size and shape, lens design, fitting measurements and budget determine whether the difference will be worthwhile.
This guide explains the numbers, what they can and cannot tell you, and how to compare options for your actual glasses, not a generic prescription chart. That matters when you are reglazing a frame you already own: the frame you send in affects the finished result as much as the material label.
What Does Lens Index Mean?
A Material Property, Not a Quality Score
The refractive index describes how strongly a material slows and bends light. Higher-index material can deliver the same corrective power with flatter curves, which may make a lens thinner. The values 1.5, 1.6, 1.67 and 1.74 describe material, not prescription accuracy, durability or overall quality.
Changing index does not change your prescription: a right-eye −4.00 D correction remains −4.00 in 1.5 or 1.67 material. See the Optometrists.org high-index guide for the definition and overview.
Why Lens Power Affects Thickness
A minus lens for short-sightedness is thinnest in the centre and thickest at the edge; a plus lens for long-sightedness is usually thickest at the centre. Cylinder (CYL) for astigmatism can make thickness uneven around the lens.
So “I’m a −5.00” is not enough to predict the finish. Sphere, cylinder and any near addition in each eye affect shape. Index changes the material’s curvature potential, not the prescription geometry.
How Do 1.5, 1.6, 1.67 and 1.74 Compare?
| Common index | What it generally means | When it may be worth discussing | Keep in mind |
|---|---|---|---|
| 1.5 | Standard index; often plastic in common ranges | Lower or moderate powers, or when a compact frame hides the edge | Often the economical baseline; may leave more visible thickness for stronger prescriptions |
| 1.6 | Mid-index; exact material varies | A possible balance when a step up from standard is useful | The visual change may be modest or more noticeable depending on frame and Rx |
| 1.67 | High index; commonly available in plastic | Stronger powers, larger frames or exposed edges where thickness matters | Costs and reflections may increase; coatings and exact material vary |
| 1.74 | Very high index; commonly available in plastic | When a specific frame and prescription show a worthwhile extra reduction | Usually a smaller incremental step from 1.67; not automatically lighter or better value |
These are comparison points, not prescription cut-offs. Two laboratories can also use different product names and material specifications. For example, Reglaze4U’s single-vision catalogue currently labels options 1.5, 1.6, 1.7, 1.8 and 1.9. Its varifocal index category also uses 1.7 and 1.8 labels. The catalogue category name alone does not confirm whether each item is plastic or mineral glass, and the displayed 1.7/1.8 values should not be treated as an exact match for every supplier’s 1.67/1.74 material. Ask for the precise index and lens substrate on the quote.
The steps are not a linear thickness ladder: a move from 1.5 to 1.6 does not promise the same millimetre change as 1.67 to 1.74. Index alone also does not tell you whether the lens is spherical, aspheric or digitally surfaced; design can affect shape and appearance. Compare the specific lens specification and predicted finish, not only the order of the numbers.
1.5: The Standard Starting Point
A 1.5 lens is not a “bad” lens. For modest powers it may look and work well, especially when a thicker rim hides edge thickness. It can be the sensible baseline if higher index brings little visible benefit.
1.6: A Possible Middle Ground
A move to 1.6 may reduce thickness, with the result depending on the frame. Compare a compact rounded shape with your preferred frame and choose from the estimated finish, not the label alone.
1.67: For Cases Where Thickness Is More Noticeable
Discuss 1.67 when your prescription and frame leave noticeable edge or centre thickness. Ask for the estimated change in your frame and confirm whether the price includes anti-reflection coating.
1.74: When the Last Step May Matter
A comparable 1.74 can be thinner than 1.67, but the added reduction may be smaller than upgrading from 1.5. Pay the premium only if an estimate predicts a visible difference that matters to you.
Why Your Frame Can Matter as Much as the Index
Size, Shape and Lens Position
A frame’s eye size, shape and optical-centre position affect the lens blank and edge. With minus powers, smaller, rounder frames often reduce edge thickness; a larger or wider lens may be thicker at the same prescription and index.
Your pupillary distance (PD) and how the frame sits determine lens centration. If its optical centres are far apart relative to your PD, extra decentration can thicken minus-lens edges. Fit still comes first: a frame that pinches or slips is not a good choice just because it uses less material. A thick, dark full rim may conceal an edge better than a thin metal or rimless frame, even when physical thickness is identical. That is a visual difference, not a thinner lens; confirm the frame can accommodate the finished lens.
Plus and Minus Prescriptions Need Different Thinking
For short-sightedness, the visible edge may matter most; for long-sightedness, centre thickness and magnification may be more noticeable. Cylinder can change the thickest point. Tell the dispenser which issue you want to solve rather than asking only for “the thinnest lens”.
Does a Higher Index Always Mean Thinner, Lighter or Better?
Thinner Is a Possibility, Not a Millimetre Guarantee
For the same prescription, frame and lens design, a higher index generally permits a flatter, thinner lens. But a table comparing index numbers cannot predict your finished thickness without the frame measurements and full prescription. A retailer’s percentage claim is meaningful only for the specific comparison behind it; do not assume it applies to your lenses.
Weight is related but not identical. Less lens volume may reduce weight, yet the density of the exact material and the size of the finished lenses also matter. If comfort is your main reason for upgrading, ask for an estimated pair weight or compare the complete glasses, not just the index number.
Reflections and Colour Fringes Are Worth Asking About
Higher-index materials can reflect more light at their surfaces than standard plastic. An anti-reflection (AR) coating can reduce this, but may add cost; confirm what the quote includes.
Abbe value relates to colour dispersion. Some high-index formulations have lower values than standard plastic and may produce colour fringes away from the optical centre. Effects vary by material and wearer. 20/20 Magazine’s high-index overview discusses this alongside reflections and centration.
Index is not an impact-safety rating. For sports, work hazards or a child’s glasses, ask about suitable material and protection rather than choosing by index alone.
Which Lens Index Do You Need? Use This Four-Step Check
1. Start With a Current, Complete Prescription
Use the prescription you intend the lenses to correct. Include both eyes and all listed values, not just the SPH column. Index cannot compensate for an out-of-date prescription. If you have noticed a change, follow professional advice and obtain an up-to-date prescription before ordering; see Reglaze4U’s article on glasses with the wrong prescription for related context.
2. Choose or Confirm the Frame
If you already own a frame you love, have its condition and suitability checked before assuming it can take every lens. If you are choosing a frame, ask to see smaller and rounder shapes as well as your preferred style. A thoughtful frame choice can sometimes do more for edge thickness than paying for the highest index.
3. Ask for a Like-for-Like Estimate
Ask the dispenser or reglazing service to compare 1.5, 1.6, 1.67 and 1.74, where offered, using your actual prescription, frame and fitting measurements. Request the estimated centre or edge thickness at the thickest point, whether figures are before or after edging, any price difference and the coating included. If a service cannot provide a millimetre estimate, ask what assumptions its index recommendation uses.
4. Compare the Benefit With the Total Cost
A price comparison should include the lens design, coatings and any fitting or reglazing charges. A higher index may be worth paying for if the predicted reduction matters to you; it may not be if the edge is hidden and the visual difference is small. Choose the lowest-cost option that meets your appearance, fit and comfort needs, not the highest available number by default.
Can You Use High-Index Lenses When Reglazing Existing Frames?
Often, but the frame must be sound and accommodate the lens thickness, shape and prescription. Deep grooves, drill mounts, strong curves, wrap styles and brittle frames can constrain the options. Have the frame checked before ordering.
Reglaze4U’s online reglazing guide describes entering a prescription, choosing lenses, sending in the frame and having new lenses fitted. Ask how the frame and selected index are checked before posting a favourite pair. Do not infer a material from marketing labels such as “thin” or “ultra thin”: ask which refractive index and lens type they identify on the order. Reusing a sound frame can be practical, but does not guarantee a specific environmental saving; see Reglaze4U’s frame-reuse article.
Index is separate from lens design: it does not tell you whether you need single-vision, bifocal or varifocal lenses. See Reglaze4U’s lens-type guide, varifocal range and glass lens categories.
A Practical Example: Same Prescription, Different Frame
Illustration, not a thickness calculation: two people have −5.00 sphere in each eye, but one chooses a small round frame near the pupils while the other chooses a wider angular frame with more decentration. Their edges may differ at the same index. Cylinder, fit, design and supplier specifications are also needed for exact figures.
Each should ask: “What thickness would each index produce in my chosen frame, and is that change worth the cost?” One may be satisfied with 1.5 or 1.6; the other may benefit from a higher index, or a smaller frame. Only a product-specific estimate can settle it.
Before You Order: Five Questions to Ask
- What exact material and refractive index is this option, rather than just its shop name?
- How thick will the lens be at its thickest point in my chosen frame?
- Does the estimate use my full prescription, PD and frame measurements?
- Is anti-reflection coating included, and what is the total price with it?
- Would a smaller or rounder frame achieve a similar appearance for less?
The Bottom Line: Choose the Lens, Not Just the Number
In short, higher index can bend light with less curvature and often allow a thinner lens for the same correction. It cannot guarantee the thinnest-looking, lightest or best-performing pair: prescription, design, frame, centration, coatings and price all matter.
1.5 may be enough; 1.6 or 1.67 may be a useful compromise, while 1.74 is worthwhile only if the estimate shows a meaningful reduction. Reglaze4U’s categories display 1.7 and 1.8 rather than exactly 1.67 and 1.74, so confirm the material. Contact Reglaze4U to ask about current options, a thickness estimate and total price.
Frequently Asked Questions
Is 1.74 Always Thinner Than 1.67?
For the same prescription, frame and design, a 1.74 material generally allows a thinner lens than 1.67. The actual difference depends on the finished lens and may not justify the extra cost in every frame.
Is 1.6 a High-Index Lens?
Retailers use “high index” somewhat differently. A 1.6 material has a higher refractive index than standard 1.5 plastic and can allow thinner lenses; ask the supplier what exact material its label refers to.
Will High-Index Lenses Make My Glasses Lighter?
They can reduce lens volume, but weight also depends on the material’s density and the size of the lenses. Ask for an estimate for the finished pair if weight is your priority.
Does Frame Size Affect Lens Thickness?
Yes. A larger lens shape and distance between its optical centre and your pupil can increase thickness, particularly at the edge of a minus lens. Frame fit and your prescription still need to be appropriate.
