We get asked a lot about the benefit of Transitions lenses. These are plastic lenses that darken when you are outdoors and lighten up when you go back indoors, to look like regular, non-tinted lenses. This can be an economical way to have a good pair of prescription sunglasses without going to the expense of purchasing a separate frame, or using clip-on sunglasses. They used to be a poorer substitute for Photogray lenses, which do the same thing but would get much darker – but are made of heavier glass. This is no longer an issue as the newest generation of Transitions will get very dark and lighten relatively quickly once you go back indoors. They are now preferred over Photogray because they are lighter in weight.
It is ultraviolet radiation exposure that makes the lenses darken. Most people prefer sunglasses while driving, which brings us to the primary drawback for Transitions lenses (and Photogray for that matter): most if not all modern car manufacturers are putting UV protection in their windshields, which means if you have a car younger than 7-8 years old, the lenses will likely not darken when you are in the car.