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photochromatic lenses. I've never used them but I wanted to try some so I didn't have to carry a second set of glasses. Problem is, most of them get reviews that make them sound shit. They don't go very dark, the don't get light enough, etc. So I think I'll stick with clear & tinted.
I have prescription sunglasses. As my prescription is so strong, I cannot have simple prescription lenses but have to have inserts. I used to have inserts in different shades until my optician suggested photochromatic lenses. They work wonderfully. Just pop them on and they protect against the sun but allow me to see when it is dark. Very impressed.
Boleo is the make.
I had some that vaguely changed colour depending on the amount of light.