Myopia, Astigmatism

General image depicting normal eye and abnormal curvature of an astigmatic eye

Results of my test on Wednesday from the CostCo Citrus Heights Pharmacy:

SRX (Spectacle Prescription)
O.D. (Oculus Dexter, my right eye):

1. SPH: +0.25
2. CYL: -1.00
3. AXIS: 0.90
4. ADD: n/a
5. PRISM/BASE: n/a
   

O.S. (Oculus Sinister, my left eye):

1. SPH: -0.50
2. CYL: DS
3. AXIS: n/a
4. ADD: n/a
5. PRISM/BASE: n/a

All measurements represented in units of dioptres, a unit that represents focusing power. More dioptres mean more focusing power--so if the numbers get big, that means you need a "stronger" prescription to see clearly.

SPH means sphere--a positive number represents farsightedness and a negative number represents nearsightedness. Why? In a farsighted eye, the curve of the cornea is flatter than the curve of a normal cornea, so the correction involves "adding" to the farsighted eye to get it back to normal. In a nearsighted eye, the curve of the cornea extends farther than the curve of the normal cornea, so the correction involves "subtracting" from that curve to bring it back to a normal curvature. The point being to get the cornea back to focusing light on the retina like a normally curved cornea does. The larger the number (positive or negative) the larger the deviation from the normal curvature of the cornea, and the blurrier the vision.

CYL means cylinder, and represents the amount of power needed to correct a cornea with astigmatism (a cornea that's oval-shaped, which causes blurring (see the picture at the top), as opposed to a normal cornea that's symmetrically round like a basketball)--DS means no astigmatism. In my right eye, I need one dioptre of focusing power correction.

AXIS, for astigmatic eyes, represents a line passing through the middle of the oval-shaped cornea--in my case through the longer side. In my right eye, I need minus-one dioptre of correction along the 0-/180-degree axis (see the Wikipedia article for an explanation of why it's not "090" like the prescription says)--which means the longer side of the oval-shaped cornea in my cases lies on an imaginary line drawn between my two eyes (the 9 o'clock/3 o'clock positions). I believe reducing the focusing power of the longer part of the oval-shaped cornea helps the cornea focus in a more spherical manner.

20/20 vision means the ability to read a character 8.84 mm high at a distance of 20 feet.