vertex distance
This is the distance between the corneal apex and the center of the correcting spectacle lens,typically 12 mm.
It is necessary to compensate for vertex distance when the refraction is greater than 4.00 D. For powers less than 4.00 D the effect is minimal and can be ignored.
In practice,one uses tables to compensate for vertex distance in the power determination.
Vertex correction means that contact lens powers are greater than spectacle powers for plus refractions and less than spectacle powers for minus refractions.
Example
Compensating for vertex distance in a high plus contact lens fitted on K:
Spectacle prescription +13.00 - 2.00 090
Keratometry 41.75 44.00
Base curve . 41.75
Adjustment for vertex distance
+ 13.00 at 12 mm is equivalent to +15.50 at the corneal plane
Final contact lens power + 15.50 diopters with a diameter of 9.7 mm
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