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Excerpt from Field Guide to Visual and Ophthalmic Optics
A lensmeter (focimeter) is a device for measuring the power of a spectacle lens. A target (typically a crossed sets of lines) is imaged through a lens of focal length f. The spectacle lens under test is placed at the rear focal point of this lens. Light emerging from the spectacle lens enters an eyepiece with an internal reticle. The user axially shifts the target until it is simultaneously in focus with the reticle. This situation occurs when the light emerging from the spectacle lens is collimated. The target position d is related to the power of the spectacle lens Φ by
The target can be rotated to align with the axes of a spherocylinder lens. When cylinder is present, only one set of target lines can be in focus. Readjusting the lensmeter will bring the orthogonal set of lines into focus. The difference in power between the two focus positions is the cylinder power of the lens. The different zones of a progressive addition lens can also be measured with a lensmeter by shifting the position of the lens in front of the aperture of the eyepiece. Modern versions of the device are automated.