Multifocal contact lenses are designed to help patients focus at any distance. While this technology can change a patient’s visual experience for the better, it’s not perfect. In a new study published in Contact Lens & Anterior Eye, researchers show that multifocal contact lenses can increase light distortion effects under low light conditions. In addition, the study finds the size and shape of the pupil correlates with the size and shape of the distortion.

The investigators looked at 14 eyes of seven contact lens patients. They used a light disturbance analyzer device to characterize the light. The team also recorded metrics such as light distortion index (LDI), best-fit circle radius and its irregularity.

Of those metrics, one in particular—LDI—was generally higher with multifocal lenses, varying from 3.7%±0.3% of the single vision lens to 6.1%± 3.3% of the multifocal center-distance design and the 5mm pupil. Larger pupils weighed in with even higher LDI values when wearing multifocal lenses, shifting from 4.5%±2.0% (in 3mm pupils) to 6.1%±3.3% (in 5mm pupils). The elliptical pupil produced the largest discrepancy in the distortion size between the vertical and horizontal directions.

The team didn’t note any difference between the center-distance and center-near designs.