In the March 2023 issue of the Journal of Cataract and Refractive Surgery, Dr. Mike Zein and colleagues published a study about the effects of masks. They were measuring astigmatism on patients, and they conducted the measurements with and without face masks on. The masks used were Halyard Level 2, Halyard Level 3, and KN95.
Interestingly, they found that the astigmatism axis, i.e. the direction of the astigmatism, varied by more than 10 degrees in 53% of patients. So it seems that masks on patients can really mess up the measurement of astigmatism!