The optical function of the deep symmetrical convexiclivate purecone fovea of deep-sea notosudids has been examined within the limits of geometrical optics, assuming, inter alia, that the full lens aperture is available for formation of foveal images, that the spherical aberration of the lens is negligible, and that the refractive index of the retina is slightly higher than that of the vitreous. Refraction at the vitreo-retinal boundary in the deep part of the fovea distorts a decentred image of a point source into an elongated spot, almost a double spot, while light from an extended source (residual daylight) will produce uneven illumination within the fovea, with a ring-shaped area of least illumination concentric with the foveal centre. It is suggested that the main optical function of the notosudid fovea is to break the camouflage of mesopelagic animals using small photophores for countershading. The theory implies the angular distribution of the photophore light to differ from that of the residual da...