**Figure 4. **Defining the anterior sector
(AS). An orthographic projection of an anterior lens quadrant is
generated. A circle (1) of diameter D is fitted by eye to the arc of
the quadrant. A square (2) with sides equal to D is fitted around the
circle. The intercept of diagonals (3 and 4) defines the center of the
circle. The fiduciary nucleus, *d*, in the constellation *a*,
*b*, *c*, *d* is identified in the projection. The
distance between *d* and the center of the circle is measured and
used as the radius of a second, smaller circle (5), concentric with the
first. An anterior sector (yellow) is drawn in the smaller circle, with
a central angle (θ) of 60°.