ASTIGMATISM, basketballs, and footballs... How are they related?

It's all about curvature! An eyeglass lens gets it's power or prescription, from the curvature that is produced on the front and back surfaces of the lens.

The front surface of the eye (called the cornea) is just like an eyeglass lens. It's curvature determines it's power. If it's curved too much, it produces a point focus in front of the retina, if it's not curved enough, the point focus falls behind the retina (refer to our previous post about Nearsightedness VS Farsightedness for more info).

                                                                                 
So far, we have only talked about one point of focus. If the cornea is spherical, like a BASKETBALL, it will only produce one focal point. This is due to the fact that no matter where light hits on a spherical object, it will encounter the same curvature, so all points on a spherical cornea will focus to the same point, either in front of, right on, or behind the retina.

The other side of the proverbial coin is a non-spherical cornea, much like a FOOTBALL. A football is not spherical. So if an eye is shaped more like a football than it is a basketball, it will form 2 points of focus. One for each curve. The curvature from tip to tip (the ends of the football) will form one focal point, and the sharper curve (the one your hand wraps around when throwing a football) will form another point.

What this means to you as a patient... both curves need to coincide on the retina to form a clear image. We can only do this by using glasses, or contact lenses (called toric lenses when correcting for astigmatism).

Next time your Optometrist brings up how your eye is shaped like a football, you will know exactly what he/she is talking about. :)

Please feel free to contact us about any of your eye-related questions. We would love to help you out.