What are the best sunglasses that lighten and darken with changing light conditions?
You're looking for photochromic lenses, which vary tint as lighting conditions change. Photochromic lenses work well, but I've found that because the lenses change slowly, they are best left at home for activities where you go in and out of shade quickly, like mountain bicycling. But for hiking or even running they’re great.
You'll also need to figure out your budget. Oakley’s Radar Path photochromic glasses, for example, come with super high-end lenses that offer great contrast and visual acuity; they also have interchangeable lenses, venting ports, and tough, lightweight frames. But they’re $240.
The next step down would be something like the Zeal Maestro sunglasses ($170), which are photochromic and polarized, making them a good choice around water or snow. They don't come with interchangeable lenses, but I’ve worn several pairs of Zeals over the years and have found them to be a good choice: the lenses are sharp, the frames solid, and the design is both stylish and practical. The Mastro’s fit medium to large heads, anyone with a smaller head will want to check out the Tensai, which are the same price and offer the same features.
The Julbo Zulu Falcons also offer both polarized and photochromic lenses, in heavier, more rugged frames. Like the Zeals, the lenses are fixed, and at $190, they work as nice multi-purpose glasses.
If you're on a tight budget check out the Tifosi Tyrant, which is a pretty good bargin at $70. Tifosi even makes a pair of orange lenses that the company says are ideal for mountain-biking. An orange lens offers different contrast and sun reduction than brown or copper lenses, but it provides great depth perception and makes the Tyrant ideal for fast-moving sports. As always, try before you buy.