Defocus curves are created by first obtaining best-corrected distance and near visual acuity for the viewer. The presbyopia correcting intraocular lens (multifocal lens) uses 'defocus curve' assessment technique in a series of positive and negative powered lenses to simulate different distances from near to far over which the subject's visual acuity (VA) is evaluated.
A defocus curve is most commonly used to assess the performance of presbyopia-correcting intraocular lenses.
The patient's best-corrected distance visual acuity is then graphed on the Y-axis at zero, which represents infinity. Minus lenses are then added to the best-corrected distance visual acuity. Viewing through the addition of a minus lens creates divergent light rays with the same refractive effect as bringing the eye chart closer to the viewer. In this manner, a -1D lens represents a 1m viewing distance. Likewise, a -2.50D lens represents 40cm, or approximately 16 inches.
The distance between the viewer and eye chart can be interchangeably plotted in centimeters, inches, or diopters. Plotting the acuity while viewing through a series of progressively more minus lenses creates the defocus curve for near objects.
- Gupta N, Naroo SA & Wolffsohn JS. Is randomisation necessary for measuring defocus curves in pre-presbyopes? Cont Lens Anterior Eye. 2007; 30:119-124.