Vertex Power Formulas
Effective Power (De)
The effective power formula tells us how the power of the lens was effected when it was moved wither farther away or closer to the eyes.
Remember to check what you are looking for first. Ask yourself if the lens moved closer or farther away and your final answer should match that.
A lens that moves away becomes more PLUS in power.
A lens that moves toward becomes more MINUS in power.
There are two ways to write the formula based on which direction the lens moves. The sign of d changes depending on which direction the lenses are moved. A lens that moves closer to the eye makes d a positive (+) number and a lens that moves away from the eye changes d to a minus (-) sign. The formulas for effective power are;
De = Dt / (1 + d x Dt) when moving away (more plus)
or
De = Dt / (1 - d x Dt)
where;
De = Effective Power
Dt = Total power of the lens (starting power)
d = movement in METERS. (must convert to mms).
Effective power with a toric lens.
Now we will be calculating the effective power of the SPH & the CYL.
You MUST use the POWER in the major meridians, DO NOT USE THE CYL Power.
Transpose/Use the Optical Cross to get your powers.