Ask An Optician…What Is The Axis On A Prescription?

//Ask An Optician…What Is The Axis On A Prescription?

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.

2015-04-24T09:36:25+00:00 June 4th, 2009|

About the Author:

have been a dispensing optician these last 14 years. Been in practice and on the road working for Carl Zeiss. Used to play a lot of golf (not any more) and now play far more online poker than I should!


  1. catalogbiz June 4, 2009 at 1:27 pm

    Thanks for answering my question Glasses Direct! Still a bit baffling though!

    You said there was no average, but what is most likely: Is having an axis of between 80 and 120 more likely than 1 or 180?

  2. David June 5, 2009 at 10:11 am

    The most common are 180 and 90. That doesn’t make them the ‘average’ just more likely to crop up.

  3. Simon June 6, 2009 at 9:05 pm

    I second 180, 90 here in the uk.

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