How to Test an Optical Lens

Eyeglass lenses come in many different prescription powers. It is possible to wear glasses with a prescription lens in one side and non-prescription lens (called plano) in the other if only one of your eyes needs corrective power. Lens power is written as a prescription and contains a sphere (sph), cylinder (cyl) and axis. Testing the power of an optical lens requires a machine called a lensometer.

Things You'll Need

• Lensometer
• Lens

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Instructions

• 1

  Place the glasses or a single lens on the platform of the lensometer. The front of the lens should be facing upward. Gently lower the lens holder down onto the middle of the lens to keep the lens in place.

• 2

  Look through the lensometer's eyepiece and turn the axis drum and power wheel until you can see the mires come into focus. The mires are three thin lines that look like a stripe. They will become clear when you have turned the power wheel to the correct power. Write down the number from the power wheel as "sphere." Sometimes there will be two sets of lines running in opposite directions. If they all come into focus at the same time, this optical lens only has spherical power and does not have astigmatism correction, called cylinder. If only one set of lines is clear, you must determine the cylinder of the lens also.

• 3

  Rotate the power wheel away from you. Three bold lines will come into focus by turning the power wheel. Look at the number on the power wheel where the second, thicker set of lines came into focus. The difference from the power of the sphere number in Step 2 will be the "cylinder" power of the lens. For example, if your sphere power was +1.00 and the second set of mires stops at +3.00 on the power wheel, the difference is called the cylinder value of the lens and would be +2.00. The prescription of the lens would be written as Sphere: +1.00, Cylinder: +2.00.

• 4

  Note the number on the axis dial, the wheel located at the base of the lensometer. The numbers range anywhere from 0 to 180 and are written on a prescription as "Axis" or "Degrees." If the axis wheel stopped at the number 135 when your second set of mires came into focus, the optical lens you tested would have the prescription Sphere: +1.00, Cylinder: +2.00, Axis: 135.