dioptry

1. A unit proposed by Monoyer to evaluate the refractive power of a lens or of an optical system. It is equal to the product of the refractive index in the image space and the reciprocal of the focal length in metres. (Symbol: D.) Thus a lens with a focal length (in air) of 1 m has a power of 1 D, one with a focal length of 1/2 m, has a power of 2 D, etc. 2. It is also incorrectly used to represent a unit of curvature, being equal to the reciprocal of the radius of curvature expressed in metres. See curvature of a surface; myodioptre; fundamental paraxial equation; refractive error; vergence.

Table D3 Relationship between dioptres and focal length (in air)
		focal length
dioptre value		(cm)		(in)
0.25		400		157
0.50		200		79
1.00		100		39
1.50		67		26
2.00		50		20
2.50		40		16
3.00		33.3		13
4.00		25		10
5.00		20		7.9
6.00		16.7		6.6
7.00		14.3		5.6
8.00		12.5		4.9
9.00		11.1		4.4
10.00		10		3.9
12.00		8.3		3.3
14.00		7.1		2.8
16.00		6.2		2.4
20.00		5.0		2.0

Table D4 Relationship between prism dioptres and degrees
prism dioptres (Χ)		degrees (º)		minutes (′)
1		0.573º		0º34′
2		1.14º		1º8′
3		1.71º		1º43′
4		2.28º		2º17′
5		2.85º		2º51′
6		3.43º		3º26′
7		4.0º		4º0′
8		4.57º		4º34′
9		5.14º		5º8′
10		5.71º		5º43′
15		8.57º		8º34