Lens shapes: (a) plano-convex; (b) bi-convex; (c) best form; (d) positive meniscus; (e) plano-concave; (f) biconcave; (g) negative meniscus. The lenses shown here have a focal length of (a)–(d) f = +100 mm (converging or positive lenses) or (e)–(g) f = −100 mm (diverging or negative lenses). The focal lengths are calculated assuming lenses made of N-BK7 glass and with a diameter of 25.4 mm (1 inch).