At my eye exam yesterday I learned that my vision does in fact need some help. Moreover, I learned that my astigmatism has returned. Here’s my prescription:
The “Spherical” column shows the refractive correction across both the vertical and horizontal axis. The “Cylindrical” column shows the correction necessary along one axis, vertical, horizontal, or diagonal. The values in the spherical and cylindrical columns are in terms of diopters. The value in the Axis column is in degrees, and shows the axis of the cylindrical correction.
A diopter is the reciprocal of the lens’ focal length in meters. If a lens has a focal length of 1/2 meters, it is a 2 diopter lens. For comparison, a +10 diopter lens (I.e., a focal length of 10 centimeters) would make a excellent magnifying glass. A human eye has a refractive power of about 60 diopters.
The correction can be expressed as a positive or negative number. A positive (like my left eye) correction magnifies, while a negative correction spreads. A magnifying glass, which is a positive lens, concentrates sunlight, while a negative lens would diffuse or spread the light out.
The spherical correction is the main correction, and the cylindrical correction is the fine tuning. In my case the correction is necessary along one axis, rather than across the entire field of view. This is known as an astigmatism. The degrees listed in the Axis column indicate along which axis the correction should occur. In my case the blur is located along the 45 degree axis in my right eye, and along the 10 degree axis in my left eye.
You can read more about this on Wikipedia’s eyeglass prescription article.
I will be getting progressive lenses in new glasses in about ten days. My distance vision is fine, but as the object I am trying to focus on draws nearer, I need some correction.
The final piece of news I got in the exam is that I have the beginnings of age appropriate cataracts on both eyes. As you age, the lens in each of your eyeballs begins to harden and become opaque. The doctor feels that my eyes are perfectly health, and that I shouldn’t worry about the cataracts for 10 or 15 years yet. In the meantime, she wants me to talk a daily multivitamin and to wear good UV sunglasses when I’m outdoors.