Under Construction: Calculators for common ABO-style problems
Use these calculators to check your work on typical ABO exam calculations. They don’t replace understanding the formulas—but they can keep you from getting bogged down in arithmetic.
How to use this page
Each calculator walks through the same steps you’d use on paper. Enter the values given in a problem, run the calculation, then compare the result to your own work.
- MBS: Frame PD, decentration, and safety factor.
- Near PD: Gerstman’s three-quarter rule from working distance.
- Dioptric power: From focal length in cm or meters.
- Prism: Prentice’s Rule with base direction.
Minimum Blank Size (MBS) calculator
Computes the approximate minimum blank size needed for a given frame and patient PD using a common ABO-style approximation: MBS ≈ ED + 2 × decentration per eye + safety factor.
- Frame PD: A + DBL.
- Total decentration: Frame PD − distance PD.
- Decentration per eye: (Total decentration) ÷ 2.
Near PD – Gerstman’s three-quarter rule
Estimates near PD from binocular distance PD and working distance using Gerstman’s three-quarter rule: Inset per eye (mm) ≈ 0.75 × dioptric demand (D).
- Dioptric demand: 1 / working distance (meters).
- Inset per eye: 0.75 × demand.
- Near PD: Distance PD − 2 × inset per eye.
Dioptric power from focal length
Converts focal length to diopters using the basic formula F (D) = 1 / f (meters). Works with focal length entered in cm or meters.
Prism – Prentice’s Rule
Computes prism using Prentice’s Rule: Δ = c × F, where c is decentration in centimeters and F is lens power in diopters.
- Enter power (D) and decentration (mm).
- Choose a base direction for context (base in/out/up/down).
- The calculator reports prism magnitude and direction.