Sometimes the old ways can be best.
They’re certainly old fashioned, but glass lenses are still the most optically efficient material for making optical lenses.
But the rumours are true. They can be incredibly heavy, bulky and yes, they can shatter like in the old movies.
Let’s delve in.
Can you still get glass lenses?
You can still have glass lenses fitted to your spectacles.
They’re the heaviest common lens material today, but give the best refractive index for their thickness. Glass lenses are also more scratch resistant than modern plastic lenses, even with protective hard coatings.
Despite their extra weight, you may prefer glass lenses compared to plastic ones, depending on your prescription and personal preferences.
Are glass lenses better?
Materialistically, glass has a higher refractive index than plastic.
Because glass is much denser, it can be made thinner than an equivalent plastic lens with the same dioptric power.
In other words, glass is more efficient at bending light than plastic lenses. Which in optics, is the holy grail material for making lenses (as long as you don’t drop them.)
If your prescription has an exceptionally strong sphere or cylinder power, the refractive index of glass can be better for minimising bulky lenses. Using the correct index, glass can normally be made thinner than a plastic equivalent.
Even if you have a mild prescription with a low spheric power, glass lenses for spectacles are mighty hard if you’re looking for durability. They’re superbly scratch resistant, even more so than modern hard-multi-coatings (HMC) on plastic lenses.
And finally, glass lenses have a great reputation.
When people ask about your stylish spectacles, they’re renowned for their optical quality and carry retro notions of being made to last. A rare thing in today’s throw away culture.
The downside of glass lenses
Famously, mineral lenses also have the capability of shattering.
Yep, just like in the movies.
Something to consider if you’re fairly active or your occupation poses impacts on your eyewear.
Furthermore, glass is much heavier at nearly twice the weight of CR39 plastic lenses. Not ideal when they could easily weigh more than your spectacles themselves.
This could happen for two reasons.
Wrong index
Compared to plastic lenses, glass lenses can be disproportionally heavy if they’re made from a low-index lens in a strong prescription. If you have a high sphere and/or a high cyl (more than plus or minus 3) you’ll require higher index lenses to avoid heavy thick lenses.
To avoid “milk bottle lenses” click here to discover your ideal lens index.
Big frame, big lenses
If you prefer wearing large style spectacles, the combined weight of two large glass lenses can also make your frame incredibly heavy. To keep the weight down, it helps if your glasses frame is slim/rectangular to minimise the bulk of glass.
Breaking news
If you regularly play sport or have a manual labour job, the fragility and weight of glass makes plastic lenses as a much safer, more forgiving lens option. If you want a lightweight frame for intense physical use, sports glasses with CR39, polycarbonate or Trivex lenses are better suited.
How much do glass lenses cost?
1.50 index, anti-glare single vision glass lenses begin at around £52 in 2024. However, depending on your prescription, coatings and the index used, glass spectacle lenses can vastly vary in price.
Other types of lenses for glasses
Polycarbonate lenses are made from plastic, known for their impact resistance and can be exceptionally thin and lightweight. They’re ideal for active use if you put high physical demands on your eyewear. Children and sportspeople favour this lens material for its hard-wearing properties and inherent UV protection.
Trivex lenses are made from plastic. They're moderately more impact resilient and lighter in weight than polycarbonate. Trivex lenses are regularly used for prescription safety spectacles for their high resilience, but cost slightly more than standard polycarbonate equivalents.
CR39 lenses are the most prevalent plastic lens material and are nearly half the weight of glass. They have the highest uncoated scratch resistance amongst all types of plastic lenses. CR39 is very easily tinted making it ideal for prescription sunglasses.
High-index lenses refers to any type of plastic or mineral lens material with a refractive index higher than 1.5 (standard index.) High index lenses can be made from CR39, polycarbonate, Trivex or glass in various mid to high indexes ranging from 1.56, 1.60, 1.67 and 1.74. Generally, the higher the index of lens, the thinner it is.
Aspheric lenses use more complex, undulating areas of curvature. They tend to have flatter curves which makes them more visually appealing than conventionally curved ‘spheric’ lens types. They can be made from polycarbonate, CR39 or high-index plastic.
Glass lenses are the oldest material for making ophthalmic lenses and can reach a refractive index as high as 1.9. The have excellent scratch resistance and can be good for especially strong prescriptions. Also called mineral lenses, they can be tinted for sun use and/or anti-glare coated. Glass however is not suitable for sporting activities due to it's susceptibility to shattering.
Polarised lenses are a sunglasses-specific lens designed to reduce glare and improve visual clarity. They use a chemical filter within or upon the lens which diminishes reflected glare from flat shiny surfaces such as water, snow or ice. They're especially popular for water-sports or driving to reduce visual fatigue over long periods of exposure. They can be CR39, polycarbonate or glass.
Photochromic lenses are characterised by their 'darkening' reaction to varying strengths of UV light. In brightly lit environments, they automatically darken to provide better visual comfort, handy if you're sensitive to bright light. In indoor environments, photochromic lenses emulate normal plastic lenses. They can be CR39, polycarbonate or glass.
Which glass lenses are the best?
Whilst glass has a high refractive index, there are various brand options available. Zeiss, Hoya, Essilor, Nikon, Seiko and Shamir etc. Depending on your prescription and lifestyle, real glass lenses may or may not be suitable.
Are glass lenses better than polycarbonate?
Both are viable lens materials. Glass is extremely scratch resistant, can provide a very high refractive index, but is more fragile to hard impacts. Polycarbonate is also hard wearing and has great impact resistance, but has a slightly lower refractive index. If you want hard-wearing lenses, polycarbonate or Trivex are safer for sport or protective prescription eyewear.
Are glass lenses dangerous?
Glass lenses can shatter upon hard impact which poses a more significant danger compared to plastic-based lens materials. Lenses made from resin polycarbonate or CR39 resin don’t shatter like this, which is they’re deemed safer, especially for sporting activities.
Are reading glasses made of glass or plastic?
Most, if not all modern reading glasses lenses are now made of plastic known as CR39. This is a hard wearing resin material that outdated glass lenses in 1940 when it was first invented by Columbia-Southern Chemical Corporation as part of their ‘Columbia Resins Project.’ [?]
It was their 39th iteration of resin that later became prolifically used for making optical lenses, hence the abbreviated name CR39.
What do glass lenses do?
According to your prescription, glass lenses create dioptric correction for the eye by bending and directing light towards the correct area of the retina. This is how all corrective lenses work to aid your near, intermediate or distance vision for prescription use.
What is glass lens called?
Glass lenses are also known as mineral lenses or crown glass lenses. These names refer to the alkali-lime silicates they’re made from instead of the commonly used polymer-based CR39 optical lenses.
A singular eyeglass lens may also be known as a monocle.
What lens material is best?
No particular lens material is “best.” This depends on your prescription, lifestyle and budget. Glass is optically superior but is also prone to shattering and can be very heavy if you have a high sphere and /or high cyl prescription.
Polycarbonate is much lighter and thinner than other lens materials but can be very expensive depending on your prescription.
Hopefully you found this article helpful. Please check out our other eyecare blogs. Thanks for stopping by.