Sport prescription sunglasses are used in a wide variety of conditions, if you are on the slopes or a mountain bike, working out at the gym or just heading to the beach for the day, the light can change dramatically as you go.
When the light dims, indoors or maybe as you go through a sheltered section of a trail, this can leave your sports sunglasses reducing visibility, and you find yourself constantly taking them on and off.
But not with Photochromic lenses, which react to light automatically, darkening to cut out bright light and protect your eyes. The reverse is true too, as you go from bright light to low light, walking indoors for instance, the lenses automatically lighten, so you can still see what you are doing.
When buying discount prescription sport sunglasses online, choosing a pair that have photochromic lenses makes sense, they are much easier to use in changing light.