Axis is the orientation, or plane of the cylinder portion of your lens, used to compensate for astigmatism. You don't have an axis value if you don't have astigmatism. From the lens wearer's perspective, 0 degrees of axis is horizontal, and increasing values move clockwise around the eye. The axis of cylinder is symmetrical, so it only goes from 0 to 180 degrees. The convention is that the horizontal is described as 180 rather than as 0.
An axis near 90 degrees is sometimes called "with the rule" and near 180 degrees "against the rule". "against the rule" is more common in young children, while "with the rule" is more common after starting school, and "against the rule" returns after the working years. It is commonly believed that the axis between your eyes is likely to have some symmetry or be 90 degrees off, but this is not true.
The axis of astigmatism can be measured with a stenopaeic slit or JCC lens but the use of these tools is complex and easily misunderstood, it's usually best to get your axis from your most recent prescription.
This section in a nutshell: Never change the axis value when reducing your prescription.
When reducing cylinder value, you should never change the axis value if it has been consistent for some time. The axis only refers to the orientation in which the cylinder is affecting your eyes, and any reduction of cylinder will happen on this orientation.
- Dobson, V.; Fulton, A. B.; Sebris, S. L. (1984-01-01). "Cycloplegic refractions of infants and young children: the axis of astigmatism". Investigative Ophthalmology & Visual Science. 25 (1): 83–87. ISSN 1552-5783.
- Kanellopoulos, Anastasios John; Asimellis, George (2015-8). "Distribution and Repeatability of Corneal Astigmatism Measurements (Magnitude and Axis) Evaluated With Color Light Emitting Diode Reflection Topography". Cornea. 34 (8): 937–944. doi:10.1097/ICO.0000000000000476. ISSN 0277-3740. PMC 4500657. PMID 26057324. Check date values in:
- Mckendrick, Allison M.; Brennan, Noel A. (1997-08). "The Axis of Astigmatism in Right and Left Eye Pairs:". Optometry and Vision Science. 74 (8): 668–675. doi:10.1097/00006324-199708000-00029. ISSN 1040-5488. Check date values in: