In order to choose sunglasses for cycling or running, you need to consider 3 aspects: lens category, interchangeable lenses and photochromic lenses.
1. Lens Category
You can cycle or run in any weather conditions. Sun, rain, fog... In order to protect your eyes while seeing clearly, choose the appropriate lens category.
Lens category: it is the index of glare protection, that ranges from 0 (the lightest lens) to 4 (the darkest lens).
There is a lens category for every weather condition!
The brighter the outdoor light, the more glare you get, and the darker the lens should be.
Conversely, the darker the sky, the lighter the lens should get to allow proper vision.
For cycling and running, it is recommended to choose category 0, 1, 2 or 3 lenses depending on the sunshine intensity.
Note that category 4 lenses must not be used for road driving. These heavily tinted lenses are only for high mountain and sea conditions, where the sun rays are more intense.
CATEGORY 3 - For sunny weather conditions. Bright outside conditions. Full UV protection.
CATEGORY 1 - For cloudy or foggy conditions. Low brightness outside. Full UV protection.
CATEGORY 0 - For rainy conditions or at night. Very low brightness outside. Full UV protection.
CATEGORY 2 - For partly sunny weather conditions. Average brightness outside. Full UV protection.
2. Interchangeable Lenses
To each day its weather. To each day its lens category. If you cycle or run in any weather, opt for sunglasses with interchangeable lenses.
Sunglasses with interchangeable lenses simply means a frame with several sets of lenses of various categories
Before going out, you just need to position the appropriate lens category onto the frame. The lenses are locked or clipped onto the sunglasses frame.
Sunglasses may be sold with one, two or even three additional pairs of lenses.
3. Photochromic or Variable Tint Lenses
Photochromic lenses are lenses with a variable tint. Depending on the light conditions, the lens automatically and continuously adjusts its tint. The lens category changes progressively, without the need to change the lenses. Because the weather can be changeable, because you alternate between shadow and sunny areas, it is not always convenient to have to choose one single lens category. With photochromic lenses, you don't have to worry about choosing the appropriate lens tint. For cycling and running, photochromic lenses usually vary from category 1 to 3, in order to adapt to as many light conditions as possible.