Defocus Curves Do Not Underestimate Visual Acuity

To evaluate the equivalence of two methods for assessing the visual acuity (VA): the method based on varying the test distance (VD) from the subject, and the defocus curve (DC) method, based on placing the chart at far distance and using trial negative lenses. Since some previous studies reported that DC underestimated VA, we aim to revisit the issue and extend the study to include young subjects with accommodation preserved and monofocal pseudophakic subjects. Our study is particularly relevant because the DC has become the gold standard method in the clinics to evaluate the depth of focus provided by modern presbyopia-compensating IOLs and compare their performances.

Group of Applied Optics and Image Processing, College of Optics & Optometry, Universitat Politècnica de Catalunya-BarcelonaTech, Spain.

Department of Ophthalmology, Consorci Sanitari del Maresme, Hospital de Mataró, Mataró (Barcelona), Spain.

Fifty subjects participated in the study in two groups: young phakics with accommodation preserved and pseudophakics implanted with a monofocal intraocular lens (IOL). Best-distance-corrected VA was measured monocularly, with the eye of better VA and under natural pupil. Each subject was tested with both the VD and DC methods. For the VD method, we used six high-contrast Landolt-C optotype charts made on purpose, consistent with the ISO 8596:2018 standard, and placed at six fixed positions from far to near. For the DC method, the chart was placed at 5m and defocus was introduced with negative lenses to simulate the six fixed vergences. The illuminance on the optotypes was constantly controlled with a photometer.

The mean best distance corrected VA at 5m was -0.18±0.07 logMAR for the young phakic group whereas 0,00±0,08 for the pseudophakic group. Mean pairwise VA values comparison at each distance showed no statistically significant differences between the two methods (VD and DC) in both groups even after applying two corrections for more accurate comparison. These corrections accounted for the diminution of minus lenses (1.2% per diopter) and the initial 5m-distance position of the test. Our results rebutted the fact that DC method underestimates VA as reported for monofocal pseudophakic subjects in other works.

The DC method, with the addition of trial negative lenses while the chart remains at a fixed distant position, reproduces correctly the VA obtained in natural conditions, when the chart is placed at various distances from the subject whose VA is being assessed. The VD and DC methods are equivalent in precision and accuracy for phakic and pseudophakic subjects and can be used indistinctly in clinical studies and in the clinical practice. However, DC is more practical than VD in the clinics because it does not require further control of the chart positioning and illuminance.