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# Lesson 4 The Diopter Unit

Formerly, lenses were measured by the focus in inches. If a lens was strong enough to focus at one inch, it was called a one inch lens. If it focused at ten inches, it was called a ten inch lens. At forty inches, it was a forty inch lens. Now, instead of measuring the length of the focus we measure the curvature which makes the focus. A forty inch focus was adopted as the unit measure of curvature. It was given the name Diopter. One diopter equals a 40 inch focus lens.

The Diopter measures the curvature power of lenses similar to the dollar which measures the purchasing power of money. Both are shortened by the inital D. or d. Both are added and subtracted, both are divided in halves and quarters. The unit of both is "one, period, double zero (1.00). One and a half is written 1.50, one and one-quarter is written 1.25.

The Diopter is added and subtracted the same as the dollar. If you have a plus 6.00 curvature on one side of the lens and a minus 5.00 curvature on the other side, it is a plus 1.00 lens. if you have a plus 6.00 on one side and a minus 7.00 on the other side, it is a minus 1.00 lens. The plus curvature lenses add curvature to your eyes. The minus curvature lenses subtract and reduce the curvature of your eyes.

If a lens is ground or molded with a plus 6.00 curvature on one side of the lens and a minus 6.00 on the other side, the curvatures neutralize each other. It becomes a neutral lens. If lenses are ground or molded with plus curvatures on one side and with identical minus curvatures on the other side, they are called plano lenses. They have no optical power. The proper name for them should be "Neutral lenses" and "Neutral glasses."