Variable neutral density (ND) filters are handy tools for photographers, in that their opacity can be adjusted simply by rotating them. So, why not use them for adjustable-tint sunglasses? Well, that's exactly what eyewear company Lombell has done.
A typical photographic variable ND filter consists of two pieces of polarized glass, arranged one in front of the other.
When the front piece is rotated to the left or right relative to the back piece, the filter's tint increases or decreases accordingly. As it increases, the brightness of the light passing through into the lens is reduced by an increasing number of f-stops – the maximum number depends on the make/model of the filter.
The titanium-frame Lombell sunglasses simply take two 9-stop variable ND filters and utilize them as their lenses. Users just twist the front piece of each lens to make it darker or brighter. Gradated markings ensure that the two lenses both get set to the same opacity.
According to the company, the glasses block over 99% of UVA and UVB light regardless of the tint setting. What's more, the back piece of glass in each lens can be replaced with a custom prescription lens if needed.
And yes, photochromic transition lenses do already change tint automatically. Some people find that they do so too slowly, however, plus they won't darken if the wearer is looking out onto a bright scene from within a shaded area (such as if they're driving a car on a sunny day).
Manually-adjustable electrochromic sunglasses are another option, although they tend to be expensive and have batteries that require recharging.
The Lombell variable ND sunglasses are available now via the company website, at a currently discounted price of US$89.99 for a regular pair or $99.99 for a set with prescription lenses.
Source: Lombell