A spectacle prescription is made up of several numbers prefixed by a plus or minus sign. Each eye will have the prescription or refraction for that eye specified in the opticians report. Here is the prescription of a person who is myopic with astigmatism.
-2.00 / -1.5 x 180
The first (+/-) sign in front of the first number refers to short or long-sightedness. As the shape of the eyeball in myopia causes the image to be focussed in front of the retina, focussing power must be subtracted to refocus the image on the retina so in this example a minus sign is present.
The first number (2.00) is the unit of optical power called a dioptre. One dioptre of optical power brings light to a point focus in one meter, two dioptres (D) in half a meter, three dioptres in 33cm. You can see that if you had an optical correction of -3.00D your natural focus would be at 33cm without glasses, this would enable you to read comfortably without glasses at any age.
The second number (1.5) refers to the amount of astigmatism. The effect of astigmatism on vision is variable but above 0.75D is thought be significant for most people. The +/- sign refers to the meridian or clock hour of the astigmatic power.
The last number which follows the multiplication sign (x 180) refers to the axis or angle of astigmatism. Most people with astigmatism will start out with a negative number followed by x 180, this is called ‘with the rule’ because it applies to the majority. A negative number followed by x 90 means that the meridian of corneal power is 90 degrees different, this is ‘against the rule’ astigmatism. A negative number followed by x 45 or x 135 refers to oblique astigmatism.
What is near ‘add’?
Your optician will measure your near vision using a standard reading type. Once presbyopia develops with loss of flexibility of the natural lens you will notice that with additional (plus) lenses you will see more comfortably. The optician will ‘add’ focussing power to your distance correction, so if the prescription above were your distance correction the new prescription might read:
-2.00 / -1.5 x 180 near +1.00
If you required an additional 1.00D of power to read comfortably.
What is prism?
If your eyes do not align perfectly there will be an additional entry in the prescription relating to prismatic correction which will be a number followed by ∆ symbol and the orientation of the prismatic lens which will be base in (BI), base out (BU), base up (BU) or base down (BD) so the prescription might now read:
-2.00 / -1.5 x 180 near +1.00 1∆ BO
If the correction was 1 prism dioptre base out.