Learn the why
Guides
Each guide walks one calculation from first principles with the canonical exam examples, then hands you off to the matching calculator and drill.
Transpose an Rx step by step
Add, flip, rotate: the three moves, the axis wrap, and how to self-check.
Prentice's rule
Where the formula comes from, base directions, and the meridian-power trap.
Why vertex distance matters
Effective power, the sign convention, and the canonical +10.00 D example.
Reading a lensometer
The two line focuses, the drum, the axis wheel, and writing down the Rx.
ABO math overview
The six calculation moves the exam leans on and a practice plan for them.
Frequently asked questions
What do the guides cover?
The calculation topics of the ABO exam and the dispensing bench: transposing a prescription step by step, Prentice's rule and base direction, why vertex distance matters, reading a lensometer, and an overview of the math the ABO expects.
Do I need the guides to use the calculators?
No. Every calculator shows its worked steps on its own page. The guides add the why: where each formula comes from, the conventions behind it, and the traps graders like to set.