Difference between revisions of Vertex distance
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"corrected" is not the same as "perceived". Also add a few more words.
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("corrected" is not the same as "perceived". Also add a few more words.) |
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The '''vertex distance''' is the distance between the surface of your eye and the center back of your lens. As the lens moves further from the eye, the perceived strength of your lenses is altered. | The '''vertex distance''' is the distance between the surface of your eye and the center back of your lens. As the lens moves further from the eye, the perceived strength of your lenses is altered. | ||
* When a plus lens is moved away from the eye, it is ''perceived'' as being stronger | |||
* When a minus lens is moved away from the eye, it is ''perceived'' as being weaker | |||
This is particularly important to know about when switching between contacts and eyeglasses, and for very high myopes: to achieve a given correction for short-sightedness, the lenses of glasses needs to be stronger than the lenses of contact lenses. | |||
==Practical Guidelines== | ==Practical Guidelines== | ||
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:If you switch from glasses to contact lenses at high myopia or vice-versa you definitely will need to account for vertex distance before buying new corrections. | :If you switch from glasses to contact lenses at high myopia or vice-versa you definitely will need to account for vertex distance before buying new corrections. | ||
:If you calculate your own correction based on blur distance from your eye, but intend to buy glasses, you will need to adjust for the vertex distance. | |||
* <div> '''I wear contact lenses as [[normalized]] and put [[plus lenses]] over them as my [[differentials]]. Do I need to worry about vertex distance?'''</div> | * <div> '''I wear contact lenses as [[normalized]] and put [[plus lenses]] over them as my [[differentials]]. Do I need to worry about vertex distance?'''</div> | ||
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<math>D_C=\frac{1}{\frac{1}{D}-x}</math>, | <math>D_C=\frac{1}{\frac{1}{D}-x}</math>, | ||
where <math>D_C</math> is the | where <math>D_C</math> is the perceived diopter number, <math>D</math> is the diopter strength of your lenses and <math>x</math> is the vertex distance in meters. It is important to note here that this equation is sensitive to minus signs of your diopter strength. | ||
Example for a vertex distance of 15mm (=0.015m): | Example for a vertex distance of 15mm (=0.015m): | ||