What they do: Lenses darken in sunlight and turn clear indoors, adapting to light so one pair works all day.
Protection & options: Block 100% UVA/UVB, can be prescription, with different darkness levels/colors; polarized versions exist.
How to choose:
Driving often? pick driving-optimized photochromic (faster in-car activation).
Glare (water/snow)? choose photochromic + polarized.
Everyday use? medium darkness; gray = true color, brown = more contrast.
High Rx? select thinner index. Wide heads? target ~160 mm frame width