If you think that pantoscopic angle and pantoscopic tilt are the same, then you’re not alone. Even optical textbooks misuse the terms. Let me attempt to clarify the difference between these two optical terms. Pantoscopic angle relates to the frame (front to temple angle—NOT WORN). Pantoscopic tilt relates to the lens and the individual position of wear (vertical angle the lens optical axis forms to the patient’s visual axis, AS-WORN.)
Pantoscopic angle describes the angle of the frame front relative to the temple. It is a frame metric, not an as-worn metric. Adjusting the pantoscopic angle can be used to straighten the frame on the face. We adjust the angle by bending the temple up or down to raise or lower the frame front on the patient’s face. We bend just one temple down to raise one side of the frame front, or we angle the temple up to lower one side of the frame front. We do this to make the frame sit straight on a patient’s face. After all, few of us are perfectly symmetrical, with both ears in perfect alignment with perfectly symmetrical facial features.
Adjusting the pantoscopic frame angle of a frame will change the lens’ pantoscopic tilt angle in front of the patient’s eyes when worn.
Pantoscopic Tilt describes the angle formed by the lens tilt in the vertical plane relative to the patient’s visual axis when worn. Pantoscopic tilt is a position of wear measurement that affects how the lens is positioned in front of the patient’s visual axis in the primary gaze position. Adding pantoscopic tilt to a lens made with the OC centered at a 90 degree angle to the patient’s visual axis will induce oblique aberrations. In this scenario, pantoscopic tilt changes the sphere power and introduces cylinder power at 180 degrees. We learn about pantoscopic tilt when we learn about Martins Rule of tilt in basic optics. We learn that pantoscopic tilt effectively shifts/raises the OC of the lens by 1 mm for every 2 degrees of tilt, which means that the line of sight is at an angle to the lens in its primary position of gaze. Martins Rule states that the optical center should be lower in the lens by 1 mm for 2 degrees of pantoscopic tilt to compensate. Easy enough for a spherical single vision lens but less so for a spherocylindrical lens or a progressive. Free-form lens design software compensates the lens surface for oblique aberrations induced by tilting the lens’ vertical plane in front of the patient’s visual axis in the primary gaze position. Compensation changes the effective power to match the prescribed power ascertained during refraction with flat trial lenses positioned at a 90 degree angle perpendicular to the patient’s visual axis.
So think “frame” for pantoscopic angle and “lens” for pantoscopic tilt.
• Deborah Kotob
Pro to Pro Director