Fig 12. In this anisometropic prescription with executive-style bifocals, the vertical imbalance encountered at the reading level has been neutralized with the application of Fresnel prisms base up on the right and base down on the left. Therefore, although the power is clearly different for the two lenses, the ruler behind the spectacles is seen to have little relative displacement. Fresnel prisms can be applied as shown as a temporary diagnostic tool prior to prescribing the final slab-off prism.