Eyeglasses consist of two separate lenses, which may have different optical properties. These properties include the axis, cylinder and sphere measurements for plain lenses, and bifocal lenses also have an add power measurement. The axis measurement is in units of degrees, and all other measurements are in units of diopters. An optometrist can measure the properties of each lens in a pair of eyeglasses with a lensometer. This requires good eye-hand coordination, since you must rotate two knobs at the same time.
Things You’ll Need:
1 Place the eyeglasses on the lensomter’s platform and center its viewing lens on one of the primary eyeglass lenses. Secure the eyeglasses into place.
2 Rotate the lensometer’s focus and axis knobs to make the small lines parallel and in focus. The reading on the focus knob is the sphere measurement. Assume for this example that the sphere measurement is +2.00.
3 Turn the focus knob to bring the fat lines into focus. Subtract the sphere measurement from the current reading on the focus knob to obtain the cylinder measurement. If the focus knob now has a reading of +5.00, the cylinder measurement for the lens is +3.00. Read the measurement on the axis knob to obtain the axis measurement.
4 Release the eyeglasses from the lensometer and reposition them so that the lensometer is viewing the bifocal lens. Secure the eyeglasses into place, and bring the fat lines into focus. Subtract the cylinder measurement from the current measurement on the focus knob to obtain the add power for the bifocal lens. If the current measurement on the focus knob is +6.00, then the add power for the bifocal lens is +1.00.
5 Repeat steps 1 through 5 with the other lens of the eyeglass to obtain the complete prescription for the eyeglasses.