Frame Selection, Fitting, and Measurement
Boxing system and frame PD
The boxing system measures the lens aperture as a rectangle. A is the horizontal box width, B is the vertical box height, ED is the effective diameter (the longest diagonal), and DBL is the distance between lenses. Frame PD equals A plus DBL. Note that A plus DBL is not the same as the distance between the geometric centers of the two eyewires when the bridge is inset.
Measuring the patient
Take binocular PD and, for progressives or high powers, monocular PDs. Segment height is measured from the lowest point of the lens (the bottom of the eyewire or the edged lens) to the top of the segment or the fitting cross. For progressive lenses the fitting cross is usually placed at or slightly above the center of the pupil.
Fitting parameters
Vertex distance of 12 to 14 mm, pantoscopic tilt of 8 to 12 degrees, face form or wrap angle matching the facial curve, temple length and bend, and even nose pad contact on both sides of the nose. The frame should not rest on the cheeks or lashes.
Selection
Frame material (zyl, monel, titanium, memory metal, stainless), bridge type (keyhole, saddle, adjustable pads), and mounting style (full rim, semi-rimless, drill mount) all affect durability and lens thickness. High minus prescriptions do best with small A and a frame PD close to the patient's PD.
Worked example
A frame has A = 52 and DBL = 18, so frame PD is 70. The patient's monocular PDs are 32 and 32, a total of 64. Total decentration is 6 mm, so each lens is decentered 3 mm inward (nasally). Trap: forgetting that frame PD larger than patient PD means inward decentration and greater edge thickness.