Not all sunglasses see the light the same way.
And that’s a good thing.

Behind every good pair of sunglasses there’s design and performance. Both should work as a team. How comfortable your eyes feel, how much glare you experience and how clearly you see the world outside all depend on the lenses you choose.

To make things simple, there are two key ideas worth understanding: lens filter categories and visible light transmission. It’s easier than it sounds.

What is Visible Light Transmission (VLT)?

Visible Light Transmission, or VLT, is just a way of saying how much light passes through your lenses and reaches your eyes. Think of it like this: High VLT lenses let in more light. They feel lighter on the eyes and work well in low or soft light conditions. Low VLT lenses let in less light. They’re darker, reduce glare and are ideal for bright, sunny days. The stronger the sun, the less light you usually want coming through your lenses.

Lens filter categories: a simple guide

To help make choosing easier, sunglass lenses are grouped into filter categories, from 0 to 4. Each category relates to how much light the lens lets through.

Category 0
Very light or almost clear lenses. Ideal for cloudy days, indoor protection, low light or when you want UV protection without much tint.

Category 1
Lightly tinted lenses. Perfect for mild sunlight, early overcast mornings or late afternoons. They offer a slight reduction in brightness while enhancing contrast, making them useful in certain sports applications.

Category 2
A versatile middle ground. Comfortable for everyday use in moderate sunlight, these lenses balance sun protection and visibility in moderate to bright sunlight. 

Category 3
The most common category for sunglasses. Designed for bright light, these are perfect for the city, the beach and everyday outdoor life. Their darker tints reduce glase and prevent squinting. 

Category 4
Very dark lenses for extreme light conditions, such as high mountains or very intense sun. These lenses are not suitable for driving, as they can be too dark to detect traffic signals and details on the road. But in harsh conditions, they’re essential. Especially due to their high-index materials with durable coatings that can withstand intense UV and environmental exposure.

FORA X ZEISS®

Choosing the right filter category is all about finding out where and how you spend your time outside. They should adapt to each one's lifestyle and routines. And here’s the important part: Lens darkness doesn’t equal UV protection. All ZEISS® sun lenses block 100% of UVA and UVB rays, no matter the category.

Good lenses do more than block light. They improve comfort, reduce glare and help you see contrast more clearly, so your eyes don’t have to work as hard.

At FORA, we choose ZEISS® lenses because they’re designed for real life outside. Different days bring different light, and the right lenses help you adapt without thinking twice. Understanding lens filter categories means making better choices. And better choices mean better days outside.