The three most common refractive errors are Myopia – Near Sightedness Hyperopia –Far sightedness Astigmatism – This is caused by an unevenness in the cornea which causes blurred vision at all distances Presbyopia – This is simply the natural deterioration of the eye’s lens due to age. Typically it is noticeable by the time a person reaches their late 30’s as it is part of the natural ageing process. Prescriptions are written in units called diopters, they are measured in intervals of 0.25 steps and will either positive (+) or negative (-) Sphere: If you suffer from Myopia( short sightedness) the prescription will be negative (e.g -1.00, -1.25,-1.50), if you have hyperopia ( long sightedness) the prescription will be positive (e.g +1.00, +1.25, +1.50). The higher the number the stronger the prescription. Cylinder: If you have astigmatism, this simply denotes the amount of diopters (or degrees) between your corneas deepest and shallowest curves.