Astigmatism - Binoculars

This is due to differences in curvature in various meridians of the refractive surfaces (lens and cornea) of the eye. For example, in the image of a cross, the horizontal line appears clearer at a different distance from the vertical line. Or put differently, the horizontal line is in focus at the same time that the vertical line is out of focus.

The human eye can also be affected by astigmatism. In such a case, the cornea does not have the normal round shape of a football but rather the oval shape of a rugby ball. This analogy should make it somewhat easier to grasp what astigmatism is!

To the binoculars glossary