Understanding Your Eye Prescription
Many people will require some sort of vision correction in their lifetime. This is usually ascertained by a visit to an optometrist where a vision test will take place, at the end of which you will receive an eye prescription. This usually consists of a grid that separates the right and left eye, sometimes noted using abbreviations of their Latin names OD (oculus dextrus – right eye) and OS (oculus sinister – left eye), and also separates near or distance vision.
The sphere (SPH) shows whether the eye is long sighted (hyperopia) and has trouble seeing close distances or short sighted (myopia) and has trouble seeing far distances. For long sighted eyes a number with a plus sign is given and for short sighted eyes a number with a minus sign is given. For example, if a prescription reads -0.25 under the right eye and +6.00 under the left eye it means that the right eye is mildly short sighted while the left eye is very long sighted. These units are dioptre (D) and they tell the lab technicians how curved your lens need to be in order to correct your vision and in what direction.
The cylinder (CYL) shows the amount of astigmatism in the eye. Astigmatism is visual distortion caused by a non-spherical shaped cornea. If this space is left empty then it means the eye is perfectly spherical and no adjustments need to be made to account for astigmatism. If a low number such as 0.25 is given, then the patient has a mild astigmatism and their eyes are mostly spherical. If a high number, such as 3.00, is given then the patient’s eyes are mostly oval in shape. This is also measured in dioptre.
The axis is also related to astigmatism. A number will only be given here if there is a number for the cylinder measurement. This measurement is not related to the vision of the patient but is instead used to help the lab technicians position the lens in the frame at the correct angle to correct the astigmatism. This measurement is given in degrees between 0 and 180.
This is a part of the prescription that is not used very often. It shows that the patient’s eyes do not actually work as a pair and when this happens, it can lead to double vision and headaches. This can be caused by muscle imbalance. The front and back surfaces of the lens are designed in such a way as to correct this visual imbalance.
This helps the lab technician position the prism in the correct place for vision correction and like the axis, is not related to the patient’s vision.
This final section of the prescription is most commonly seen in people over 45. As we get older, it can sometimes become harder to see things close to us. This ADD measurement is referred to as a reading addition and it shows that the patient has a different prescription for distance vision and for reading. This would usually require the use of bifocals or varifocals.