The eyes of cuttlefish (Sepia officinalis) have a modified horizontal slit-pupil with a distinctive W-shape in bright light, while in darkness the pupil is circular. Two suggestions have previously been made for a function of the W-shape: (1) camouflaging the eye; (2) providing distance information. Since neither of these suggestions can fully explain the function of this pupil across the entire visual field, particularly the frontal and caudal periphery, we re-addressed the question of its functional significance. We took infra-red images of the eyes of live S. officinalis at different light intensities and from different viewing angles. This allowed us to determine the shape and light-admitting area of the pupil for different parts of the visual field. Our data show that the W-shaped pupil projects a blurred "W" directly onto the retina and that it effectively operates as vertical slits for the frontal and caudal parts of the visual field. We also took images of the natural habitat of S. officinalis and calculated the average vertical brightness distribution in the visual habitat. Computing a retinal illumination map shows that the W-shaped pupil is effective in balancing a vertically uneven light field: The constricted pupil reduces light from the dorsal part of the visual field significantly more than it reduces light from the horizontal band. This will cut the amount of direct sunlight that is scattered by the lens and ocular media, and thus improve image contrast particularly for the dimmer parts of the scene. We also conclude that the pupil provides even attenuation along the horizontal band, whereas a circular pupil would attenuate the image relatively more in the important frontal and caudal periphery of the visual field.