How to Find the Optical Center of a Lens

The optical center of a lens is the point where light passes through the lens and does not deviate, but travels straight. Spectacle lenses are either concave to correct for nearsightedness, or convex to help with farsighted vision. Both types of lenses have an optical center that is placed directly in front of the pupil. Care should be taken when determining the optical center of the lens.

Things You'll Need

• Lensometer

Show More

Instructions

• 1

  Turn the dial on the power drum on the lensometer until all the mires, the power lines, come into focus when no lens is in the machine. Adjust the focus of the eyepiece and turn the power drum dial to zero, making sure the lines are all in focus.

• 2

  Place the glasses on the lens table of the lensometer. Position the frame so the front surfaces of the lenses are facing up toward the lensometer. Release the trigger-grip lens holder on the lensometer and slide it down gently to hold the lenses in place on the table.

• 3

  Turn the power drum wheel until the first set of solid mires becomes clear. Most lensometers have a solid line or a set of three solid lines, or mires, for the spherical power of the lens and a set of three striped mires for the cylindrical power. The three striated lines go in the opposite direction of the solid lines.

• 4

  Turn the power wheel away from you to check for cylinder if there is any. Cylinder is used to correct for astigmatism in a prescription lens. Turning the wheel will bring the set of three striped lines into focus if cylinder is present. Return the wheel back to the original set of solid mires.

• 5

  Move the lens slightly if necessary until the intersection of the lines is in the very center of the eyepiece view. Release the self-recoiling lens marker and dot the lenses. The marker will make three dots in a row. The middle dot is the optical center of the lens.