Cylinder lenses are used to focus light into a line or to change the aspect ratio of the image. The focal length of a plano-convex cylinder lens is positive, as it is made of one flat surface and one convex cylindrical surface. The focal length of a plano-concave cylinder lens is negative, as it is made of one flat surface and one concave cylindrical surface. Cylinder lenses are ideal for applications in laser processing systems, R&D, and other fields.