Making one or more surfaces in an optical design aspheric is a common way to improve optical performance without increasing the number of surfaces. However it is not always apparent which surfaces will most benefit from aspherization.

Further, adding an aspheric surface can increase costs in both manufacture of the element and in tolerancing to assess the sensitivity of the system to perturbation of the aspheric surface. Therefore, more is not always better: the designer must carefully assess which surfaces to make aspheric, and generally will want to use the least degree of asphericity needed to give the desired performance.

ZEMAX's Find Best Asphere tool is a useful way to identify which surface will benefit most from aspherization. In a manner similar to Test Plate fitting, the tool replaces spherical surfaces with aspherical surfaces of user-specified degree. The user can run the tool multiple times, varying the degree of asphericity each time, and can decide whether to keep the suggested asphere or discard it.