Vertex Distance and Calculations R.D.Gopinath Supervisor – Optical Sales

Introduction Vertex distance is from the back surface of the spectacle lens to the front surface of the eye. The vertex distance affects the effective power of the lens, especially in higher powered (>4.0D) prescriptions. The standard vertex distance is about 12mm.

Lens Effectivity Lens optical effect may vary with vertex distance. Vertex distance responsible for decrease of vision. If moves away from eye, + lens becomes stronger. -- lens becomes weaker.

VD changes & the effect in ‘+’ lens Increasing the vertex distance of plus lens will increase the effective power of the lens. Decreasing the vertex distance of a plus lens will decrease the effective power of the lens.

VD changes & the effect in ‘—’ lens Increasing the vertex distance of a minus lens will decrease the effective power of the lens. Decreasing the vertex distance of a minus lens will increase the effective power of the lens.

Significance of VD In prescription must have the same effective power as the refraction test. The vertex distance of the phoroptor / trial frame must match the VD of the spectacle lenses. A vertex distance becomes significant if the diopter power of the prescription exceeds 4.0D.

Vertex compensation power A spectacle is placed at 20mm away from the eye instead of 12mm, so what is the actual prescription? To determine this we need to use the vertex compensation formula: Dc= Compensated Power Dl= Original Lens Power d= Change in Vertex Distance in Meters.

Vertex compensation power A spectacle power of -5.00 D Sph 6mm further from the eye than it should, then > The best sphere is – 4.85 D.

Vertex Compensation Formula The formula for the needed compensation per mm.of displacement, per diopter of lens power, is as follows: = D x D / 1000 The answer is multiplied by the mm of displacement. The result is added or subtracted from the diopter power according to the following set of conditions:  1)  + lens moving closer - add to increase the diopter power 2)  + lens moving farther away - subtract to reduce the diopter power 3)  -- lens moving closer - subtract to reduce the diopter power 4)  -- lens moving farther away - add to increase the diopter power.  

Example –spherical power lens Consider a -12.00 Sph refracted at 13mm.  The lens in the patient's new glass will sit 10mm away from the patient's eye.  (12 squared = 144), 144/1000 = 0.14 The movement is 3mm closer to the patient's eye, with a minus lens.   3 x .14 = 0.42D, so 0.5 D is subtracted from -12.00 to reduce the lens power to -11.50 D.

Example – cylindrical power lens If the Rx has a significant cylinder power (at least 1 D), we must perform the calculation for the primary meridians of power. A +12.00D S/ +3.00x180 lens, refracted at 14mm & Spec’s on pt’ face 10mm away. For +1200: 12 squared is 144, 144/1000 =0.14 mm.The movement is 4mm closer to the patient's eye, with ‘+’ lens.  4 x .14 =0 .48, so 0.5 D is added to +12.00 to increase the lens power to +12.50D.  For +15.00: 15 squared is 225, 225/1000 = 0.225  4 x .225 = 0.9, so 1 D is added to +15.00 to increase the lens power to +16.00.  Our adjusted Rx is:  +12.50D S /+3.50 x 180*.

How to measure VD? To determine VD use Distometer. Distometer places one arm on the eye lid while the other is placed on the back of the lens, and a small scale attached to the device measures the distance.