How to Determine a Glasses Prescription

A glasses prescription is determined by placing the lenses on a machine called a lensometer. This type of optical equipment determines a glasses prescription by reading the powers of the lenses. The lensometer is a bit challenging to use at first but knowing the basic steps of using a lensometer makes determining a spectacle prescription easier.

Things You'll Need

• Lensometer
• Pair of glasses

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Instructions

• 1

  Place the eyeglasses on the platform or table facing you, arm pieces pointing away. Lower the lens holder down onto the optical center of the right lens gently to keep the lenses from moving.

• 2

  Look through the eyepiece as you rotate the axis drum and power wheel until the mires---three thin solid lines in one direction---come into focus. Record the number from the power wheel as sphere.

• 3

  Turn the power drum until the three fat lines come into focus. These will be the cylinder power of the lens, if there is cylindrical power. If the three fat lines and three thin lines came into focus in Step 2, there is no cylinder power, and the lens will have only a sphere power and thus no astigmatism correction.

• 4

  Note the numbers on the axis drum where the second, fat set of lines came into focus. If there is a cylinder correction, there will be a number that corresponds to where the thick lines focused. Record the number where the axis drum stopped, which will be anywhere between 1 and 180 degrees.

• 5

  Repeat with the left lens. One or both lenses can be either simple, with just a spherical power, or complex, with spherical and cylindrical and axis numbers.