Neutral density (ND) filters are used by photographers to stop down the amount of light entering the camera lens. The filter is actually two layers that are polarized; rotate the outermost one, and the admitted light is reduced or increased.A startup called Lombell figured it would be a good idea to put this arrangement into a pair of sunglasses:While I can appreciate that this must've sounded like a cool idea, wouldn't the UX suffer, since each lens needs to be adjusted independently? I've never tried these, but unless your eye can easily detect between F-stops, I'd guess you have to take the glasses off as you dialed them to ensure symmetry. If this is you, these are $90.