Introduction to Lensometry
Susan Brummett, COMT
Little Rock, Arkansas
January 2007



  • Lensmeter – instrument designed to measure the prescription of an optical lens.
    • Also defined as an instrument to determine the spherical power, cylinder power with axis, and optical center of a lens.
  • Mires –measurement images viewed by technician.•Reticle –eyepiece or ocular adjusted for examiner’s focusing point.•Instrument consists of an ocular for viewing the mires, a flat stage or table for supporting the spectacle frame, a power dial, and an axis wheel.
  • Sphere –lens with optical power being the same in all meridians (expressed in diopters).

  • Cylinder –lens that has different refractive/optical power in each meridian. It is used to correct astigmatism.

  • Axis –meridian of cylinder with the minimum power perpendicular to maximum power meridian. Expressed in units of degrees.

Steps to Neutralizing Spectacles

  • Adjust the eyepiece
    • Adjustment is critical for obtaining accurate measurement
    • Turn power drum to plano
    • Turn ocular to most PLUS position (counterclockwise)
    • Slowly turn ocular toward minus until the numbers and circles appear sharp and focused.
    • STOP!!!!
  • Check calibration of lensmeter
    • Rotate the diopterdial until all mires come into focus without a lens on the frame table.
    • If the diopterdial reads “0”once all mires are in focus, the lensmeteris properly calibrated.
  • Position spectacles
    • Place glasses on platform so that both lenses are resting on stage with the temples facing away from you (for distance RX)
    • This prevents movement of the lenses and inaccurate reading of the axis
  • Locate optical center of lens
    • Beginning with the right lens (as with most ophthalmic procedures) center the lens on the platform and lock in place with the lens holder.
    • Upon viewing the lens through the ocular, the mires should be centered. If not, possibly have ground-in prism present.
Read Full Article