In terms of optics, the iris functions as an aperture stop. The entrance pupil of the eye is the image of the iris aperture stop that the cornea can image. The exit pupil of the eye is the image of the iris aperture stop that is formed on the retinal side of the ocular lens. The human eye is not a rotationally symmetric, or centered optical system; its optical parts are decentered. This follows from the optical element surfaces that may be toroidal and also tilted with respect to each other. Instead, the human has a visual axis, which is the line that passes through the fovea, back nodal point, front nodal point, and the point of fixation on the object. The visual and the optical axes of the human eye are tilted from each other by 5° on the object side. Photopic (high luminance) vision depends on the photoreceptor cones. The cones contain three different types of pigments: there are S, M, and L cones, each type with its own spectral sensitivity under high luminance or photopic conditions. Therefore, the cones can discriminate various wavelengths. Schematic eye models are a useful tool for the designers of optical systems in both ophthalmology and optometry. A well-known application is the design of devices for correcting the errors of refraction. This application includes corrective glasses and contact lenses. Schematic eye models are also useful for the surgical planning of laser photorefractive surgery to alter the curvature of the cornea with the result of correction of refractive errors. Another application is in the design of optical diagnostic medical instruments.