What is Vertex Distance?
Vertex Distance is the distance from the front of the cornea to the back of the lens, the abbreviation for it should be VD or CVD. Possible values between 7 and 30mm. If no value is given, an average value of 14mm will be taken by default.
A basic method for measuring the vertex distance is to first adjust eyeglasses and fit frames and remove the lenses without disturbing the adjustment. Ask the patient to close his eyes and raise his eyebrows (to stretch the lid tissue a bit). Place a PD rule with the end ground off to the “0” mark through the eyewire at the position of the fitting cross. The vertex distance can then be estimated from above by noting the distance at which the rule passes through the eyewire. One millimeter should be added to account for the thickness of the lid, and a small additional amount can be added if a lens with a steep back curvature is being fitted
The frame front should be positioned as close as possible to the eye without the lenses touching the brow, lashes or cheeks. This provides the widest viewing areas in all parts of the lens. Proper vertex distance also helps eliminate back surface reflections on the lenses. To obtain the correct vertex distance, adjust the nose pads on a metal frame with nose pad pliers to bring the frame front closer to or farther away from the face. When fitting a plastic frame, it is necessary to start with the proper bridge fit. The importance and the effects of vertex distance will be discussed in later modules.
How Vertex Distance affect effective power?
Prescription lenses in eyeglass frames have different effective powers at different distances, tilts and wrap angles from the eye. That means that the same prescription on two different people, one wearing lenses farther than the other would actually have different powers. Lenses become more plus (less minus) when moved away from the eye (increasing the vertex) and less plus (more minus) when closer. That explains why telling a progressive or bifocal wearer to slide their glasses to the tip of their nose provides a stronger add to read the phone book or a medicine bottle.
Let¹s look at our example.
OD: -5.00 SP
OS: -5.50 SP
This patient comes in wearing glasses that are sitting 20mm away from the eye instead of 14mm, so what is the actual prescription? To determine this we need to use the vertex compensation formula:
Dc = D1/(1 + d X Dl)
Dc= Compensated Power
Dl= Original Lens Power
d= Change in Vertex Distance in Meters
For the right eye we have a spectacle power of -5.00 SPH sitting 6mm further from the eye
than it should.
Dc = 5.00 /(1 + 0.006 X 5.00)
The new sphere power is -4.85. The best way to determine vertex distance is though the use of an special instrument. Different company offers different solutions on this. Normally, this device 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.
Tags: Optician Knowledge