Lens category at a glance
- Standard
- ISO 12312-1:2022
- Most hiking
- Category 3
- Snow and glacier
- Category 4
- Driving
- Category 0 to 3 only
What do the numbers 0 to 4 mean?
Each category is a band of visible light transmission, the share of daylight the lens lets reach your eye. ISO 12312-1 sets the bands: category 0 passes more than 80 percent, 1 passes 43 to 80, 2 passes 18 to 43, 3 passes 8 to 18, and 4 passes 3 to 8.
Makers often print the transmission figure itself, as VLT, which is how you can tell a pale category 3 from a very dark one. A lens that passes 9 percent and one that passes 17 are both category 3, and you will see the difference on a snowfield.
Which lens category is right for hiking?
Category 3 for most sunny trails. Category 2 suits forest, haze and overcast days, when a darker lens makes roots and rocks hard to read. Category 4 is for glaciers, big snowfields and high altitude, where light reflects up at you from every surface and snow blindness becomes a real risk.
The same standard says a category 4 lens must not be used for driving, which matters if you drive to the trailhead in the pair you hike in. A photochromic lens that moves between categories on its own is the way around carrying two pairs.
Does a darker lens block more UV?
No. Lens category measures visible light, and UV protection is a separate property of the lens material and coatings. The American Academy of Ophthalmology is explicit that darkness says nothing about UV blocking. A pale lens can block all of it, and a dark one with no UV filter can be worse than none, because your pupils open wider behind it.
How much light does each lens category let through?
| Category | Visible light through | Use |
|---|---|---|
| 0 | More than 80 percent | Clear or cosmetic tint |
| 1 | 43 to 80 percent | Light tint, low sun |
| 2 | 18 to 43 percent | Forest, haze, cloud |
| 3 | 8 to 18 percent | Bright sun on most trails |
| 4 | 3 to 8 percent | Snow, glacier, altitude; no driving |
Bands from ISO 12312-1, the international standard for general-use sunglasses.
What this means when you're choosing
- The category or VLT printed on the lens spec, not just a tint name
- Category 3 for sunny trails, category 4 only for snow and altitude
- A separate statement of 100 percent UVA and UVB or UV400 protection
- A photochromic lens if your days mix forest and open ground
The common mistake
Buying the darkest lens for the most protection
A category 4 lens makes forest trails gloomy and footing hard to read, is unsuitable for driving, and blocks no more UV than a lighter lens with the same UV rating. Darkness is about comfort in bright light. UV protection is a separate line on the label.
Common questions
Can you drive in category 4 sunglasses?
No. ISO 12312-1 says category 4 lenses are not suitable for driving, because they let through too little light to read the road and traffic signals safely. Wear a category 3 pair in the car and change at the trailhead.
Is a category 3 lens dark enough for snow?
For a winter day hike mostly in trees, usually yes. For long days on open snow, above treeline or at altitude, a category 4 lens with side shields is the safer choice, because reflected light off snow is what causes snow blindness.
Where this is defined
- ISO 12312-1:2022, Sunglasses and related eyewear, Part 1: Sunglasses for general use, International Organization for Standardization
- How to choose the best sunglasses, American Academy of Ophthalmology
- Photokeratitis (snow blindness), American Academy of Ophthalmology