Diopter value determines the curvature degree (optical power) of spherical lens and is expressed in PWR or DPT units. The human eye lens has a certain optical power. If a person suffers from any eye defect, the curvature degree is insufficient (thus the lens is not able to achieve sufficient correction). Such eye defects must be corrected by glasses or contact lenses that have an optical power equal to the power of the eye lens so the person can see clearly. The diopter value shows how much the lens needs to be corrected to achieve quality vision. It is understood that the higher the value of the diopter, the worse the eye defect. Diopter value is marked by + or -, which determines whether the lens is for farsightedness (+) or nearsightedness (-). Contact lenses are commonly made in values from +20.00 to -20.00 dpt.