Calculation Practice
Master essential optical calculations with interactive practice problems. Build confidence for the ABO exam and real-world dispensing.
Choose a Calculation Type
Or try a mixed practice session:
Tips for Calculation Success
- • Memorize the formulas - Write them out daily until automatic
- • Watch your units - Convert mm to cm for Prentice's Rule, mm to m for vertex
- • Check sign conventions - Plus/minus matters for transposition and cylinder
- • Practice under time pressure - ABO exam is timed
- • Round appropriately - Lens powers to 0.25D, measurements to practical values
Essential Formulas Quick Reference
🔄 Transposition
New Sphere = Old Sphere + Old Cylinder
New Cylinder = Opposite sign
New Axis = Old Axis ± 90°📏 Vertex Distance
Fc = Fs / (1 - d × Fs)
d = vertex distance in meters🔺 Prentice's Rule
Prism (Δ) = c × F
c = decentration in cm
F = lens power in diopters🎯 Vogel's Rule
Base Curve = (Sphere / 2) + 6
(Use algebraic sphere value)👓 Decentration
Total Dec = Frame PD - Patient PD
Per Lens = Total Dec / 2
Frame PD = A + DBL🔍 Magnification
SM = 1 / (1 - d × F)
Plus lenses magnify
Minus lenses minify