What is the vertex distance? vertex distance calculator

What is vertex distance ?

vertex distance calculation for contact lens is very important and we may be used chart for calculate, but we can calculate also with the given value or row value. 

This is the space between the corneal apex and therefore the center of the correcting spectacle lens, typically 12 mm. 

it's necessary to catch up on vertex distance when the refraction is bigger than _4.00 D.

 For powers but 4.00 D the effect is minimal and may be ignored.

 In practice, one uses tables to catch up on vertex distance within the power determination. 

Vertex correction means contact powers are greater than spectacle powers for plus refractions and fewer 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 (on K,so no lacrimal lens effect)

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