Most vision research focuses on visual acuity as the benchmark for outcomes—but everyone knows other factors can play a significant part in a patient’s ability to see. Researchers in Korea have taken a closer look at a tried-and-true visual aid almost everyone uses at some point: squinting.

Before this study, little was known about the impact of squinting on astigmatism, and what literature does exist doesn’t agree, according to the researchers. To solve this problem, they created a model eye linked to a wavefront analyzer. They then tested four different refractive statuses: -1.50D and -3.00D of both with-the-rule (WTR) and against-the-rule (ATR) simple myopic astigmatism.

They discovered the squint—created with a 2mm horizontal slit placed in front of the model eye—caused a hyperopic shift of approximately +6.69µm with -1.50D WTR, but -2.01µm for -1.50 ATR.

Give the horizontal nature of squinting, it’s no surprise “it can improve distance vision in WTR astigmatism and near vision in ATR astigmatism,” the researchers noted.

They also found that squinting decreased astigmatism and increased aberrations in the ATR astigmatism groups but had the opposite effect for emmetropia and WTR astigmatism: it increased astigmatism and decreased aberrations.

“This pseudoaccommodation effect can cause an overestimation of near visual acuity when squinting is not prohibited,” the researchers noted in their paper.