Diopter

Japanese: diopter

…
[Adjustment]
The point at which you can see clearly when your accommodation is at its maximum is called the near point, and the point at which you can see clearly when accommodation is at its minimum is called the far point. The distance between the near point and the far point is the range at which you can see things clearly, and this is expressed in diopters (often abbreviated as D, which is the unit of lens measurement. It is expressed as the reciprocal of the focal length, with 1D referring to a lens with a focal length of 1m) and is called the accommodation power.

From [Glasses]

...In addition, for people with astigmatism, cylindrical lenses are usually included in reading glasses as well, but one idea is to omit them when the astigmatism is weak.
[diopter]
The unit of lens strength used worldwide today is the diopter (Dioptrie in German, denoted by the symbol D), which is the reciprocal of the focal length measured in meters (so a lens with a focal length of 50 cm would have 1/0.5 = 2.2 diopters). As an exception in the case of eyeglass lenses, the distance from the vertex of the second surface of the lens to the second focal point is considered to be the focal length, and the strength expressed based on this is called vertex power and is distinguished from principal point power.

*Some of the terminology that mentions "diopter" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

Japanese:

…
【調節力】
　調節力を最大にはたらかせたときにはっきり見える点を近点といい，調節を休ませたときにはっきり見える点を遠点という。近点と遠点との距離は，ものをはっきり見ることのできる範囲であり，これをレンズの単位であるジオプターdiopter(Dと略記することが多い。焦点距離の逆数であらわし，1Dは焦点距離1mのレンズをいう)であらわしたものを調節力という。…

【眼鏡】より

…また乱視のある眼の場合は近用眼鏡にも円柱レンズを含ませるのが普通であるが，弱い乱視のときは省略するのも一つの考え方になっている。
［ジオプトリー］
　レンズの強さを表す単位は，今日では世界的にジオプトリーdiopter(ドイツ語ではDioptrie,Dの記号で表示する)が用いられているが，これはメートル単位で計測した焦点距離の逆数で得たものである(したがって，焦点距離50cmのレンズは1/0.5＝2,2ジオプトリーとなる)。眼鏡レンズにおいては特例としてレンズの第二面頂点から第二焦点までの距離を焦点距離とみなしており，それにもとづいて表示される強さを〈頂点屈折力〉と呼んで主点屈折力とは区別している。…

※「diopter」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典第２版について | 情報

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