 An alternative explanation is that a partially constricted circular pupil shades the peripheral zones of the lens which would lead to poorly focused images at relevant wavelengths.The vertical slit pupil allows for use of all wavelengths across the full diameter of the lens, even in bright light.It has also been suggested that in ambush predators such as some snakes, vertical slit pupils may aid in camouflage, breaking up the circular outline of the eye.