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JA Martin, A Roorda; Optical and Visual Benefit of Multizone Bifocal Contact Lenses . Invest. Ophthalmol. Vis. Sci. 2002;43(13):971.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: The coupling of the eye’s monochromatic aberrations with the different refracting zones of a multizone bifocal contact lens have been found to alter and sometimes eliminate their intended bifocal function. We investigated the visual performance of these lenses and computed the expected variability in response for different eyes. Methods: The monochromatic aberrations of the eyes of 15 subjects (< 2.0 D sphere, < 0.5 D cylinder error) were measured with a Shack-Hartmann wavefront sensor. The wavefront of the optical zones of 3 different bifocal contact lens designs were numerically added to the wave aberration of the eyes. The modulation transfer function (MTF) and Strehl ratio were calculated for object positions ranging from infinity through the focal position of the near add. We also measured the CSF for five of the subjects with and without an Acuvue bifocal contact lens (1.5 D add). The CSF was measured for 4 object vergences from distance through the near add position. The subjects were cyclopleged (to simulate presbyopia) and viewed the sinusoidal target through a 5 mm artificial pupil. For both objective and subjective measurements, we computed the «bifocal benefit’, which we defined as 1 - (MTF or CSF with CL)/ (MTF or CSF without CL). Results: Responses to the bifocal lenses were mixed. In the numerical simulations, some eyes demonstrated bifocality while others only showed an increase in their depth of focus. In all cases, the bifocal lens offered a benefit in MTF for near vision at the cost of a decrement in distance vision. Similar benefits and decrements were observed in the CSF measurements. For distance vision, the average bifocal benefit in contrast sensitivity for spatial frequencies from 1 - 16 c/d was -0.18 +/- 0.26, representing a loss. For the near target at 1.50 D the average bifocal benefit was 0.90 +/- 1.2, representing an improvement. The largest overall benefit was observed for an eye that was expected, from the calculations, to have no apparent bifocal vision but only an increase in depth of focus. Conclusion: Multizone bifocal contact lenses do not always provide a bifocal response. However, even without a bifocal response, there is still an increase in depth of focus, resulting in an overall benefit for viewing near targets.
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