Getting Glasses Isn't Just About Your Prescription – A Complete Guide to Lens Refractive Index and Coatings

When getting glasses, salespeople always say "higher refractive index is better" and "pay extra for coatings for clearer vision." But in reality, a high refractive index isn't right for everyone, and coatings can be genuine or fake. This guide helps you understand lens specs and avoid unnecessary spending.

1. Refractive Index: What a Higher Number Really Means

The Essence of Refractive Index

The lens refractive index determines how thick the lens will be for the same optical power.

Simply put:

Higher refractive index → stronger ability to bend light → thinner lenses for the same prescription
But higher refractive index → more chromatic aberration → increased image distortion

Common Refractive Index Comparison

Refractive Index	Thickness	Suitable Prescription	Characteristics
1.50	Standard	Under -6.00 D	Lowest cost
1.56	Thinner	Under -8.00 D	Good value
1.60	Thin	Under -10.00 D	Mainstream choice
1.67	Very thin	Under -12.00 D	Recommended for high prescriptions
1.74	Ultra-thin	Over -12.00 D	Only worthwhile for very high prescriptions

How to Choose Refractive Index

Under -6.00 D:

1.56 is already thin enough
1.60 is an unnecessary expense
1.74 is a complete waste of money

-6.00 D to -10.00 D:

1.60 is a reasonable choice
1.67 looks better but has a noticeable price premium

Over -10.00 D:

1.67 is a basic requirement
1.74 is genuinely valuable

Uneven Prescription (large difference between eyes):

It's recommended to choose the higher refractive index for both lenses
Otherwise, the thickness difference between the two lenses will be obvious

2. Abbe Number: Measuring Chromatic Aberration

What is Abbe Number

The Abbe Number measures a lens's chromatic dispersion:

Higher number → less dispersion → clearer, more accurate vision
Lower number → more dispersion → possible colored fringing (chromatic aberration) at edges

Relationship Between Refractive Index and Abbe Number (Typical Values for Resin Lenses):

Refractive Index	Typical Abbe Number
1.50	58
1.56	36-38
1.60	36-42
1.67	32
1.74	33

Note: The Abbe Number for 1.56 refractive index is actually lower than for 1.50, meaning its optical quality is generally poorer.

Practical Impact

With lenses that have a low Abbe Number, you might see colored fringing around high-contrast edges like curtain edges or window frames.

Low prescription (under -4.00 D): Abbe Number effect is not noticeable
High prescription (over -8.00 D): Low Abbe Number makes chromatic aberration more apparent

3. Coatings: Understanding the Add-Ons

Essential Coatings

Anti-Reflective (AR) Coating:

Reduces reflected light from the lens surface
Improves light transmission
Present on almost all legitimate lenses
This is a basic feature, not an extra charge item

Anti-Smudge Coating:

Reduces fingerprint and oil adhesion
Good anti-smudge coatings clean easily with one wipe
Poor anti-smudge coatings require repeated wiping

Optional Coatings

Blue Light Blocking Coating:

Blocks some high-energy blue light (380-500nm range)
Lenses have a slight yellow tint (because blue light is filtered out)
Actual effectiveness is controversial:
- Screen blue light intensity is far lower than sunlight
- Evidence for blue light blocking affecting sleep is insufficient
- Mainly recommended for people who stare at screens for long periods and feel eye strain

Photochromic Lenses:

Transparent indoors, darken outdoors
Principle: Contains silver halide compounds that change color when exposed to UV light
Disadvantages:
- Car windshields filter UV light, so darkening effect is poor while driving
- Darkening speed decreases with temperature (slower in winter)
- Darkening effect weakens with long-term use

Polarized Lenses:

Filters reflected light (polarized light) in a specific direction
Very effective at reducing road glare while driving
Allows you to see underwater when fishing
Mainly suitable for sunglasses, not everyday glasses

Coating Quality Identification

Hardness Test (not recommended to perform yourself; ask the seller):

Good coatings have a hardness of 6H or higher
Poor coatings scratch easily

Reflection Color:

High-quality AR coatings reflect green or blue-green light
This is a normal phenomenon of the coating process
It's not a problem; it's a hallmark

4. Lens Materials

Resin Lenses (Mainstream)

Low density, lightweight
Can be tinted, suitable for sunglasses
Good impact resistance
Not heat-resistant (can deform in a hot car)
Not chemical-resistant (alcohol can damage the coating)

Glass Lenses

High hardness, scratch-resistant
Fewer refractive index options
Heavy weight
Safety risk if broken
Now almost obsolete

Polycarbonate (PC) Lenses

Extremely high impact toughness (commonly used for children's and sports glasses)
Refractive index of 1.59
Low Abbe Number (around 29), high chromatic aberration
Not suitable for high myopia prescriptions

5. Progressive Lenses

Who Needs Them

People over 40 who start developing presbyopia while also having myopia or needing to see both far and near are suitable for progressive lenses.

Progressive Zone Layout

Top: Distance zone (for seeing far away)
Middle: Transition zone (intermediate distance)
Bottom: Near zone (for reading)
Sides: Distortion zone (blurry)

Adaptation Period for Progressives

First-time wearers need 1-2 weeks to adapt
You need to learn to move your head to use different zones
When going downstairs, looking at your feet can put you in the distortion zone, which is dangerous initially

Internal vs. External Progressives

External progressive (front surface): Traditional process, lower cost
Internal progressive (back surface): Closer to the eye, less distortion, wider field of view, higher price

6. Frame Impact on Lenses

Pupillary Distance (PD) and Optical Center

The distance between your eyes (PD) varies per person, and the optical center of the lens must align with your pupil.

Consequences of an off-center optical center:

Creates a prism effect
Long-term wear leads to eye strain and headaches
In severe cases, affects vision development (in children)

Ask the store for the PD measurement record from your fitting

Frame Size and Thickness

The larger the frame, the thicker the lens edge (especially for myopia):

For the same prescription, small-frame lenses are much thinner than large-frame ones
For high prescriptions (over -10.00 D), small frames are especially recommended
Popular large round frames are very unfriendly to high prescriptions

7. Buying Pitfalls to Avoid

Price Traps:

Frames and lenses are usually priced separately
Ask clearly if the price includes coatings and what kind
Ask about the refractive index and brand origin

Special Considerations for Children's Lenses:

Prioritize PC material (high impact toughness)
Don't chase the thinnest option (prescriptions are usually not very high yet)
Get regular prescription checks (every six months to a year)

Online Glasses Ordering:

After getting an eye exam at a physical store, you can order lenses with the same specs online
However, progressive lenses and high-prescription lenses are better ordered from a physical store
You need accurate PD data

8. Summary

Refractive Index Choice:

Under -6.00 D: 1.56 is sufficient
-6.00 D to -10.00 D: 1.60 is suitable
Over -10.00 D: 1.67

Coating Priorities:

AR coating is essential; anti-smudge coating is a must-have
Blue light blocking is optional, not mandatory
Don't be fooled by "multi-layer coating" – ask specifically what coatings are included

Other:

Small frames are much better for high prescriptions than large frames
Ensure accurate PD measurement during fitting
If you feel discomfort or dizziness after getting your glasses, get them rechecked promptly