The optical function of acrylic cylinders used in keratoprostheses is demonstrated by using a water-filled minicamera as a model of the aphakic human eye. Lengths and diameters of the optical cylinders are important factors influencing the visual fields. The dimensions to be chosen, depends on the thickness of the cornea and the supporting tissues. The most suitable combination of visual field, magnification and diameter of the retinal image field is obtained with optical cylinders with a concave posterior surface. Using a fixed radius of curvature of the posterior surface, the radius of the anterior surfaces are calculated to make the eye emmetropic. With such a cylinder of length 4.5 mm and diameter 2.1 mm a circular visual field of 48" is obtained. With cylinder length 6.0 mm the visual field is at least 35'. The retinal image is about 190/0 larger than that of the normal phakic eye. Cylinder length exceeding 6.0 mm require a greater diameter to provide an adequate visual field. With cylinder lengths up to (3.0 mm and diameters not less than 2.1 mm, the diameter of the retinal image field is at least 12.0 mm. Accidental obliquity of the optical cylinder or off-center implantation give the possibility of undesired projection of the image field outside the macula. The obliquity or degree of off-center implantation tolerance is calculated. The retinal image may be improved by darkening the side of the cylinder.