dioptre

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dioptre

A measure of lens power. The reciprocal of the focal length in metres. A lens of 1 m focal length has a power of 1 dioptre. A lens of 50 cm focal length has a power of 2 dioptres. One of 33.3 cm focal length has a power of 3 dioptres. Dioptres are used by opticians in preference to focal lengths because they can immediately be added together or subtracted during vision testing.

dioptre

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.
Fig. D3 Effect of an 8 Χ prismenlarge picture
Fig. D3 Effect of an 8 Χ prism

Table D3 Relationship between dioptres and focal length (in air)
focal length
dioptre value(cm)(in)
0.25400157
0.5020079
1.0010039
1.506726
2.005020
2.504016
3.0033.313
4.002510
5.00207.9
6.0016.76.6
7.0014.35.6
8.0012.54.9
9.0011.14.4
10.00103.9
12.008.33.3
14.007.12.8
16.006.22.4
20.005.02.0

Table D4 Relationship between prism dioptres and degrees
prism
dioptres (Χ)
degrees (º)minutes ()
10.573º0º34′
21.14º1º8′
31.71º1º43′
42.28º2º17′
52.85º2º51′
63.43º3º26′
74.0º4º0′
84.57º4º34′
95.14º5º8′
105.71º5º43′
158.57º8º34