Astigmatism

The eye is not shaped like a spherical tennis ball, but more like a football

The curves on the eye are different when measured up/down versus left/right

The Rx will need a different strength going up/down than it will need going left/right

We can correct this with eyeglasses, contact lenses or vision (refractive) surgery (for adults)

Astigmatism tends to be stable over the years

Hoya Vision: https://www.hoya.com/en/eye/astigmatism/

Mayo Clinic: https://www.mayoclinic.org/diseases-conditions/astigmatism/symptoms-causes/syc-20353835

Cleveland Clinic: https://my.clevelandclinic.org/health/diseases/8576-astigmatism