show the reciprocal form (hope it's correct !)
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(show the reciprocal form (hope it's correct !)) |
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In the above example the -4.0 dpt glasses yield the same level of correction as -3.75 dpt contact lenses. It can be seen that vertex distance '''increases''' the strength of [[Plus Lenses]] and '''decreases''' the strength of [[Minus Lenses]]. The effect is noticeable above 4.0 dpt and is mostly negligible for [[Low Myopia]]. | In the above example the -4.0 dpt glasses yield the same level of correction as -3.75 dpt contact lenses. It can be seen that vertex distance '''increases''' the strength of [[Plus Lenses]] and '''decreases''' the strength of [[Minus Lenses]]. The effect is noticeable above 4.0 dpt and is mostly negligible for [[Low Myopia]]. | ||
The expression looks less intimidating when you remember that (by definition) the [[Diopter]] is the reciprocal of the focal length. So it's really just: | |||
<math>f_C = f - x</math> | |||
where <math>f_C = 1/D_C</math> and <math>f = 1/D</math> | |||
Conceptually, the focus length is reduced (power is increased) because it has moved closer to the source of the image. | |||
==References== | ==References== | ||