Hey everyone, does anyone know the formula for vertex compensation? i lost my formula in all my paperwork! while im at it, can someone help me out with the formula for slab off? ive searched the internet, but none of the ones ive found look correct! Thanks!
:hammer:
I dont know if this will help, but I just finished a lab tech class and this is what they gave us
quoting
"When a lens is moved away from the eye it increases in effective plus power. When a lens is moved closer to the eye it decreases in effective plus power. Efective power is the poer of the lens after it has been moved. A rule of thumb is that a 10.00 D lens will change .10D per mm of horizontal shift. This means that a +10.00 D lens will have an effective poer of +10.50 D if moved 5mm from the eye. This same +10.00 D lens will have an effective power of +9.50 if moved 5mm closer to the eye. Moving a minus lens would have the opposite effect. a -10.00 D lens will have an effective power of -9.50 D if moved 5 mm closer to the eye and -10.50 D if moved 5 mm further away.
Examples
a 10 D lens changes .10 D per mm of shift.
a -10 D lens moved 2.5 mm closer to the eye would require a compensated power of -9.75 D
- 2.5 mm x .10 = .25D
- -10.00 + .25 = -9.75 D
I know it aint much, but hope it helps
Jason:o
The exact formula for vertex distance compensation is:
New Power = Old Power / (1 + Change * Old Power)
where Change is the increase in vertex distance in meters (use a negative value for a reduction in vertex distance.
The approximate formula is:
New Power = Old Power - Change * Old Power^2
For lenses with cylinder power, the power of each principal meridian should be compensatedly individually (that is, compensate the sphere power and then the sphere + cylinder power). The new cylinder power is the difference between the two compensated principal meridian powers.
For instance, given a prescription of +4.00 -1.00 x 180 (+3.00 through the cylinder meridian) with a refracted vertex of 13 mm and a fitted vertex of 20 mm, which represents an increase in vertex distance of 0.007 m, the new compensated prescription should be:
New Sphere = 4.00 / (1 + 0.007 * 4.00) = +3.89 D
New Cylinder Meridian = 3.00 / (1 + 0.007 * 3.00) = 2.94 D
New Cylinder = +2.94 - 3.89 = -0.95 D
New Prescription = +3.89 -0.95 x 180
I also have a vertex distance calculator available online in the OptiCampus Calculators.
Darryl J. Meister, ABOM
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