Fitting & Verification
Fitting Methods

Dyer Chart Method:

Contact lenses may also be fit according to a chart. One widely used method is through the use of the Dyer chart. Here the fitter would follow the following procedure.

Base Curve: locate the flattest corneal meridian in the left hand column of the chart. Then locate the amount of corneal astigmatism in the appropriate column to the right. Note the base curve indicated.

Diameter, Optical zone, and Peripheral Curves: once the base curve is de- termined, refer to the Dyer chart for other lens parameters and find the base curve in the left hand column. Note the other parameters indicated.

Power: To calculate the correct power of a rigid contact lens, first convert the spectacle prescription to minus cylinder form. Then drop the cylinder and axis completely. If necessary, compensate the power for vertex distance. The resultant sphere power would then correspond to the power of the lens. This power calculation assumes the lens is being fit �on K.� If the base curve varies from the flatter corneal meridian, that difference in diopters must be taken into account when considering the power of the contact lens.

Hartstein Modification:
This method stresses the value of taking measurements to determine final dimen sions of the contact lens. It uses two basic parameters to arrive at the dimensions of the contact lens�corneal diameter and pupillary size. Lens Diameter: To determine the lens diameter, add 4 mm to the size of the pupillary diameter. For example, a pupil with a diameter of 4.3 mm would require a lens size of 8.3 mm. In the case of larger corneas, 12 mm or more, add 0.5 mm to the overall diameter, for smaller corneas, 10 mm or less, reduce the diameter by 0.5 mm. Optic Zone: The size of the optic zone is simply 1.5 mm smaller than the overall lens diameter. Secondary Curve: Should be 0.75 mm wider than base curve and 5.00 D flatter. Thickness: The standard thickness for a plano lens is 0.16 mm. For each diopter of minus power, 0.01 mm is subtracted from the center thickness. For each diopter of plus power, 0.02 mm is added to the center thickness. Base Curve: The base curve is fit 0.25 to 0.50 D steeper than K. It there is between 0.50 and 2.00 diopters of corneal astigmatism, a spherical lens is ordered parallel to the flattest corneal meridian, or �on K.� If the corneal astigmatism falls between 2.00 and 3.00 diopters, it would require a lens 0.50 D steeper than K. Any astigmatism over 3.00 D would indicate the use of a toric contact lens.