The term "aspheric" means non-spherical but this simple definition could include toric surfaces, which is not intended in this paper. Instead the term "aspheric" will cover only rotationally non-spherical surfaces. Aspheric surfaces occur frequently in ophthalmic optics. For example the four refracting surfaces of the eye are more accurately described by aspheric mathematical functions than by a simple spherical function. Throughout the ophthalmic literature a number of different methods for specifying aspheric surfaces have been in use, particularly in the case of ophthalmic lenses. The purpose of this paper is to review the nature of aspheric surfaces, their mathematical description and their occurrence and significance in ophthalmic optics. Their effect on aberrations is of particular significance and their effect on the oblique astigmatism in spectacle lenses will be discussed in depth
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