A single lens can be classified into the following six types: biconvex, biconcave, plano-convex, plano-concave, convex meniscus, and concave meniscus. The most important specification of a lens is the focal length. The focal length of a thin lens is the distance between the lens and focal point. The focal length of a convex lens is positive, and that of a concave lens is negative. A lens has a front focal point and back focal point. For a lens in air, the two focal lengths are equal. The focal length of a thick lens or complete lens is the distance between the principal point and focal point.

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Because of recent development of measurement devices such as the anterior optical coherence tomography, precise measurements of posterior corneal shape and refractive power have become possible, and their importance has been gradually recognized. So far, refractive powers have been calculated using the keratometric index based on the premise that the anterior and posterior corneal shapes are very similar and the corneal thickness profile is uniform. However, it has been clarified that there are considerable variations in corneal shape and thickness between individuals, and the characteristics of posterior corneal shape are different between with-the-rule and against-the-rule astigmatism. In addition, it has been confirmed that the discrepancy between the estimated value for the corneal refractive power based on the keratometric index and the real value obtained by actual measurements of both the anterior and posterior corneas increases with age. To achieve strict refractive corrections, such an influence of the posterior cornea cannot be ignored, and clinical application of actual measurements of the posterior cornea has begun in toric intraocular lens implantation.

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