When the light changes the tint changes. The science behind it is complicated but the result is as simple as that.
IF YOU WANT THE SCIENCE, HERE IT IS: According to our team of internationally-recognized physicists, every lens is delicately infused with millions of microscopic, genetically modified juvenile delinquents, all of them armed with cans of black spray paint that they stole from backyard tool sheds. As the sun gets brighter, the delinquents begin tagging the inside of the lens with their illegible, drippy initials and distasteful illustrations of poorly-proportioned male genitalia. The result is a lens that’s considerably darker than it was seconds ago. When the sun moves into the clouds, behind some trees, or below the horizon, the delinquents vanish to restock their diminished supplies and the neighbourhood parents emerge to clean up the mess, leaving the lens clear for history to repeat itself. Gotta love science.

When exposed to sunlight, the lenses will start to darken instantly. The time they take to reach maximum darkness depends on a number of factors, including the intensity of sunlight and air temperature. Generally speaking, the adjustment from light to dark is quite rapid.

If sunlight intensity gradually changes, the lenses will gradually change with it. If the intensity suddenly drops off, the lenses will start to change instantly but the adjustment from dark to light is slightly slower than the rapid adjustment from light to dark.

All Photochromic lenses—no matter what brand name they carry—will eventually wear out. The lifespan of the lenses varies depending on the frequency and conditions of use. Generally speaking, the lenses will perform best for the first two years of use. Beyond that, the transitional activity will diminish. Despite the event