How eyes focus
Astigmatism
Optical glossaryHow eyes focus
An eye whose optics are curved unequally in different directions, so light focuses along two lines rather than one point. Extremely common and fully correctable with a cylinder.
Illustrative · the shape of the principle, not measured data
In more depth
An astigmatic eye is curved more steeply in one direction than the other, like the back of a spoon rather than a ball. Light through the steep direction focuses sooner than light through the flat one, so a point becomes two short lines at different depths and nothing is quite sharp at any distance.
A little is almost universal. It is corrected with a cylinder set at the right axis, and it tends to be felt as smeared letters, glare around lights at night, or fatigue rather than obvious blur. Larger amounts are where lens centration and a level frame matter most.