Diopter represents an optical imperfection of the eye, i.e. the inability of the eye to sharpen the image. You are short-sighted if you see clearly the objects in close proximity, while the distance is blurred (for example, you do not recognize a person coming towards you, you can not drive without glasses or watch TV). In this case the image focuses in front of the retina (inner coat of the eye sensitive to light stimuli) and you need negative diopter value in order for the image to focus directly on the retina and to be sharp.

You are farsighted if you do not see clearly distant nor close objects (for example, you do not recognize a person coming towards you, you can not drive without glasses or watch TV, but you also have blurred vision when reading, working on a computer, writing messages on your phone - your eyes strain or you have headaches working at short distance). In this case the image focuses behind the retina and you need positive diopter value in order for the image to focus directly on the retina and to be sharp.

You have astigmatism if both close and distant objects are blurred and have a shadow. Astigmatism refers to an irregular curvature of the cornea.