The six moves
| Skill | Formula | Drill |
|---|---|---|
| Transposition | S' = S + C; C' = -C; axis rotates 90 | practice |
| Prentice's rule | P = c x F in the meridian of decentration | practice |
| Vertex compensation | F' = F / (1 - d x F) | practice |
| Meridian power | F = S + C x sin^2(theta - axis) | practice |
| Minimum blank size | ED + 2 x decentration + allowance | practice |
| Vertical imbalance | Per-eye Prentice at the reading drop; difference | practice |
Supporting knowledge
Two table-reading skills ride along: the ANSI Z80.1 tolerance brackets, drilled with the tolerance checker and summarized in the reference table, and the base-direction conventions for induced prism. The spectacle magnification power factor appears occasionally and reuses the vertex denominator.
A practice plan that works
- One skill per sitting until its drill streak reaches ten with the worked steps feeling redundant.
- Switch to the mixed drill so formula selection itself gets practiced.
- Finish with timed exam runs. Time pressure is a separate skill; rehearse it before exam day, not on it.
- Keep the formula sheet printed beside you until you notice you have stopped looking at it.
Units, the recurring villain
Prentice wants centimeters, vertex wants meters, blank size stays in millimeters, and the axis lives on a 1 to 180 scale where 180 is written instead of 0. Most wrong answers on calculation questions are unit slips, which is exactly what repetition drills out.