Coatings are where a lot of the money goes in a good lens, and where most of the failures happen. They are layers measured in fractions of a wavelength of light, and each one adds a capability and a way for the lens to wear out.
Anti-reflective, and why it belongs on the back
An anti-reflective coating is a stack of thin films tuned so that reflections from each layer cancel each other out. Less light bounces off the lens surface, so more passes through and contrast improves.
On sunglasses the important application is the back surface, and it is the one buyers rarely ask about. Light arriving from behind and above you strikes the inside of the lens and reflects into your eye – the faint ghost image and the washed-out feeling you get with the sun behind your shoulder. Back-surface AR removes it.
Front-surface AR on a sunglass lens is less useful, and on a mirrored lens it is contradictory – the mirror is there to reflect.
AR is also the coating that fails most visibly. When it degrades it crazes into a fine web or lifts in patches, and the scatter that produces is worse at night than an uncoated lens would be.
Mirror coatings
A mirror coating is a reflective metallic layer on the front of the lens. It reduces the light entering the lens in the first place, which is genuinely useful in extreme conditions – snow, open water, high altitude – and it lets a lens be very dark without needing a very heavy tint.
The trade-offs are practical. Mirror surfaces show every fingerprint and every wipe mark, they scratch more visibly than a plain lens because the damage interrupts the reflection, and the coloured varieties shift how you perceive colour.
Hydrophobic and oleophobic layers
These repel water and oil respectively, and they sit on the outside of everything else. Water beads and runs off rather than drying into spots; skin oil and cosmetics wipe away with less pressure.
That last point is the real benefit. A lens that cleans with less force is a lens that gets scratched less over its life, so the coating protects the coatings underneath it.
It is also the first layer to wear off, typically within a couple of years. When a lens that used to shed water starts holding droplets, that is what has gone.
Scratch-resistant is not scratch-proof
A hard coat is applied to soft lens materials – polycarbonate in particular is impact-resistant but scratches easily – to raise surface hardness.
It raises the threshold. It does not remove it. Silica dust is harder than any consumer lens coating, which is why dry-wiping a dusty lens still causes damage on a hard-coated lens. Treat the claim as a margin of safety, not permission.
What destroys coatings
- Heat. The layers and the lens beneath expand at different rates. Sustained heat – a dashboard, a radiator – separates them, and crazing is the result.
- Ammonia. Household glass cleaner strips AR stacks.
- High-concentration alcohol. Tolerable once, degrading with repetition.
- Dry abrasion. The cumulative one. Every dry wipe on a dusty lens.
- Salt and sunscreen left on the surface to dry.
More coatings is not better
Each layer is another interface that can delaminate, and coating stacks fail as a stack. A lens sold on a long list of treatments is not automatically a better lens than one with a well-applied back-surface AR and a decent hard coat.
Application quality matters more than the count. It is also invisible at the point of purchase, which is one of the honest arguments for buying lenses from makers with something to lose.
What to ask for
- Back-surface anti-reflective. The most useful coating on a sunglass lens, and the least advertised.
- A hard coat, especially on polycarbonate.
- An oleophobic top layer, because it reduces how hard you have to clean.
- A mirror only if you are genuinely in high-glare conditions and accept the maintenance.
And treat coatings as consumable. They are the part of a lens with a service life measured in a few years of real use, which is an argument for buying frames that can be re-lensed rather than lenses that must last forever.