It has often been my personal experience, when out walking or riding in a vehicle in bright sunlight, that there is a need for a special type of sunglasses that would respond quickly, ideally in real-time or near-real-time to changes in light and shadow.
Let’s say I am going for a walk (this has actually happened to me) on a pathway or roadway where there are intermittent trees, in bright sunlight. One moment I will need full-on sunglasses. A few seconds later I will be in shade and sunglasses will not be necessary or even welcome. The same has happened when I have been riding in a car. Light / shadow / light / shadow. Or when it is partly cloudy: clouds — sun — clouds -sun etc.
Also, sometimes, there is a need for a special pair of sunglasses that will just block out the disk of the sun itself, but nothing else. For instance, when riding in a car headed toward a low, bright sun; such as near sunrise or sunset.
Ideally such a pair of glasses would have a variable lens, which could change in real-time to light (or shadow) impinging on various portions of it, and a black-out disk to block direct view of the sun itself.