THE 1,2, 3 OF PRISM
Here's some basics about prism in lenses
1. Eyewear with prism, means that the optical centers of the lenses do not occur at the PD. For example, if the left eye has 2 prism diopters Base Out prescribed, the point on the lens that is 2 Base Out is located at the PD.
2. As a result, in eyewear where there is prescribed prism, we find the prescribed prism values at the PD.
3. Using a manual lensmeter, the reticle is your key to locating prism.
4. Using the target, the sphere and cylinder lines, place the center of the target (where the center of the sphere and cylinder lines cross) at the center of the reticle. That is the point of no prism, the optical center. See the illustration to the right.
5. Next, move the center of the target to the point of prescribed prism and dot the lens. Voila, that's the point from which the PD is measured.
So, if there is prescribed prism in both eyes, the place dotted on each lens at the point of prescribed prism should be equal to the patient's PD.