An optical unit of power. When preceeded by a plus sign indicates that the patient is longsighted and that the rays of light passing through the lens will converge on a point of focus.The wearer when unaided can see distance but not near. A minus sign indicates being shortsighted which is opposite to the above with the rays diverging as it passes through. One dioptre is equal to one metre of focal length while two dioptres is only half a metre.

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