Lens (optics) divergence of a collimated beam, it does alter the width of the beam. The magnification of such a telescope is given by which is the ratio of the output beam width to the input beam width. Note the sign convention: a telescope with two convex lenses ("f" > 0, "f" > 0) produces a negative magnification, indicating an inverted image. A convex plus a concave lens ("f" > 0 > "f") produces a positive magnification and the image is upright. For further information on simple optical telescopes, see Refracting telescope § Refracting telescope designs. Cylindrical lenses have curvature in only
Lens (optics) object off the optical axis of the lens is imaged, where rays pass through the lens at an angle to the axis θ. Rays that pass through the centre of a lens of focal length "f" are focused at a point with distance from the axis. Rays passing through the outer margins of the lens are focused at different points, either further from the axis (positive coma) or closer to the axis (negative coma). In general, a bundle of parallel rays passing through the lens at a fixed distance from the centre of the lens are focused to a ring-shaped