Which four values are needed to determine the minimum blank size (MBS)?

• A. Patient PD, Frame PD, Decentration, ED
• B. Frame PD, Vertex distance, Inset, ED
• C. Pupil distance, Frame PD, Decentration, A constant
• D. Patient PD, Pupil distance, Frame PD, ED

Answer: (A)

The horizontal size you need for a lens blank is set by how the eyes see through the frame and where the optical centers must lie. You must know the patient’s PD to know the width between the eyes that the lenses must cover. The frame PD tells you how wide the frame’s optical centers are, which defines how much of the lens will be centered over the eyes when mounted. Decentration tells you how far the actual optical centers of the lenses must be shifted relative to the frame’s centers to line up with the patient’s pupils. ED, the effective diameter of the blank, defines the largest circle you have available on the blank; your required width must fit within that diameter.

Vertex distance and inset aren’t needed to determine MBS because they influence other aspects of spectacle optics or frame fit, not the basic horizontal width required to create a lens blank that can be edged to fit the frame.