Asking for questions on twitter the other day, this one came in:

What does the Axis part on the prescription mean? What is “average”?

I asked another David, one of our team of dispensing opticians:

“The axis is the orientation of the astigmatism. The value is in degrees, and can be anything from 1 to 180. There is no average value and the number doesn’t have any relevance to the size of the astigmatism (that is indicated in the CYL box).

It’s probably best described as being like a compass bearing. The bearing you walk on doesn’t relate to how far you walk, it’s just the direction of travel.”

And an astigmatism? What’s that?

“Instead of being spherical like a football, most eyes are cylinder shaped, a bit like a rugby ball, and need correcting in two meridians; a spherical (SPH) component, and a cylindrical, or astigmatic (CYL) one. The AXIS is the direction along which the cylindrical component (astigmatism) lies.”

So there you go, hope that helps out. How’s the language? Understandable?

The way that I imagine it (from a non-optician’s viewpoint) is like this:
Imagine holding up a tall glass (like this one) up in front of your eye. The ‘Cyl’ value is similar to the thickness of the glass, and the ‘Axis’ value is the angle at which you hold the glass in front of your eye.

Anything else you want to know, just ask in the comments.