Photochromic glasses have lenses that change tint depending on the weather or surroundings. The benefit of photochromic lenses is that in dull, overcast weather or darker surroundings (like running or cycling through woods or early / late in the day) they will lighten. But in bright, sunny conditions they will go dark. Photochromic lenses are most popular with cyclists, runners, skiers, tennis and outdoor reading enthusiasts.