A multifocal lens has 2 or more refractive elements. The power of each segment is prescribed separately.
Determining the Add Power of a Bifocal Lens
The information necessary to prescribe bifocal lenses includes (1) an accurate baseline refraction, (2) the accommodative amplitude, and (3) the patient’s social or occupational activities that require near-vision correction (eg, reading, sewing, or computer use).
Measuring accommodative amplitude
Any of the following tests can provide useful information for determining the accommodative amplitude: (1) the near point of accommodation with accurate distance refractive correction in place, (2) the accommodative rule (eg, with a Prince rule [Royal Air Force Rule]), and (3) the use of plus and minus spheres at near distance until the fixation target blurs. Binocular amplitude of accommodation is normally greater than the measurement for either eye alone by 0.50–1.00 D.
Near point of accommodation A practical method for measuring the near point of accommodation is to have the patient fixate on a near target (usually small print such as 5-point or Jaeger 2 type print) and move the test card toward the eye until the print blurs. If the eye is emmetropic (or rendered emmetropic by proper refractive correction), then the far point of the eye is at infinity and the near point can be converted into diopters of amplitude.
This method is subject to certain errors, including the apparent increased amplitude resulting from angular magnification of the letters as they approach the eye. In addition, if the eye is ametropic and not corrected for distance, the near point of accommodation cannot be converted into diopters of amplitude. In the following examples, each eye has 3 D of accommodative amplitude:
A person with emmetropia would have a near point of 33 cm and a far point at optical infinity.
A patient with an uncorrected 3.00 D of myopia would have a near point at 16.7 cm because at the far point of 33 cm, no accommodation is needed.
A patient with an uncorrected 3.00 D of hyperopia would have a near point at infinity because all of the available accommodation is needed to overcome the hyperopia.
Accommodative rule Amplitude of accommodation can be measured with a device such as a Prince rule (Royal Air Force Rule) (Fig 4-24), which combines a reading card with a ruler calibrated in centimeters and diopters. Placing a +3.00 D lens before the emmetropic (or accurately corrected ametropic) eye places the far point of accommodation at 33 cm, and the near point is also brought closer by a corresponding 3.00 D. The amplitude is then determined by subtraction of the far point (in diopters) from the near point (in diopters).
Method of spheres Amplitude of accommodation may also be measured by having the patient fixate on a reading target at 40 cm. Accommodation is stimulated by the placement of successively stronger minus spheres before the eye until the print blurs; accommodation is then relaxed by the use of successively stronger plus lenses until blurring begins. The difference between the 2 lenses is a measure of accommodative amplitude. For example, if the patient accepts −3.00 D to blur (stimulus to accommodation) and +2.50 D to blur (relaxation of accommodation), the amplitude is 5.50 D.
Figure 4-24 Prince rule.
(Courtesy of Tommy Korn, MD.)
Range of accommodation
Determining the range of accommodation, like measuring the amplitude of accommodation, is valuable in ensuring that the prescribed bifocal add power meets the patient’s visual needs. The range of accommodation measures the useful range of clear vision when a given lens is employed. For this purpose, a measuring tape, meter stick, or accommodation rule may be used.
Selecting an add power
Determine the amount of accommodation required for the patient’s near-vision tasks. For example, reading at 40 cm would require 2.50 D of accommodation. From the patient’s measured accommodative amplitude, allow one-half to be held in reserve. This reserve allows for some comfortable movement should the patient move the reading material either closer or farther away from the optimal reading distance. For instance, if the patient has 2.00 D of accommodation, 1.00 D may be comfortably contributed by the patient. (Some patients may use more than one-half of their available accommodation with comfort.) Subtract the patient’s available accommodation (1.00 D) from the total amount of accommodation required (2.50 D); the difference (1.50 D) is the approximate additional plus lens power (add) needed.
Place a lens with this add power in front of the distance refractive correction, and measure the range of accommodation (near point to far point of accommodation in centimeters). Does this range adequately meet the requirements of the patient’s near-vision activities? If the accommodative range is too close, reduce the add power in increments of 0.25 D until the range is appropriate for the patient’s requirement. Because binocular accommodative amplitude is usually 0.50–1.00 D greater than the monocular measurement, using the binocular measurement generally guards against prescribing an add power that is too high.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.