Calculating combined prism for spectacle prescriptions

1. Combined prisms are required when correction incorporates vertical and horizontal strengths in one lens. 2. Calculations of the resultant prism power requires diagramming the vertical and horizontal vectors and drawing the resultant diagonal vector. The power is either derived from the Pythagorean Theorem or measured on graph paper. 3. The resultant prism base direction or axis designation is calculated from an equation or measured directly off the diagram with a protractor.