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For most of the twentieth century, the lensometer (also known as the lens meter or vertex meter) changed very little. However, in the 1970s a number of automatic lens analyzers appeared, which eliminated almost all human involvement and quickened the determination of new prescriptions.

In this chapter, the basic principles of the traditional lensometer are reviewed to outline its strengths and weaknesses, and thus help to appreciate the usefulness of the automatic devices.

The lensometer does not measure the focal length of the unknown lens. It measures the vertex power, which is the reciprocal of the distance between the back surface of the lens and its secondary focal point, this distance being known as the back focal length.

A simple lensometer (Fig. 2-10-27) is an optical bench that consists of an illuminated, moveable target, a powerful fixed-field lens, and a telescope eyepiece focused to infinity.[25]The key element is the field lens; without this, to measure the merest …