Ever wonder why your contact lens prescription is different than your eyeglass prescription? The Answer (one of them): Vertex Distance.
Vertex distance is the distance between the back of your eyeglass lens and your eye. A contact lens sits directly on your eye, so the vertex distance is essentially zero. Your glasses and the phoropter (the instrument the doctor puts in front of you with lenses to test your vision) typically sit 14-18mm in front of your eyes. That small distance creates a difference in power and magnification between your eyeglasses and contacts. The same is true for people who like to pull their reading glasses down on their nose. This effectively increases the magnification provided by the lenses; a reason why it’s important when being fitted for new glasses that all measurements are taken or calculated for the position in which you plan to wear your glasses (high on your nose or further away).
Magnification, material, curvature, and power are all reasons why vision is different with contacts and glasses. Your eye doctor makes the necessary calculation to provide you with the most accurate prescription….Yet another example of that algebra class being put to good use!