The drops work by selectively acting on the muscles that control pupil size, constricting the eyes’ aperture. As anyone who knows photography is aware, this increases the depth of field (a larger range of distances from the eye will be approximately in focus). Of course this also reduces the brightness on the retina, so things will be darker, an effect confirmed by the linked FDA data. In that data it’s claimed that the perceived dimness is “temporary”; I guess because of accommodation that kicks in after a while. So this is like increasing your f-stop and your ISO.