Formula

1/f = (n − 1) × (1/R₁ − 1/R₂)

Use convex radii as positive and concave as negative (relative to the incoming light). f in mm returns power in diopters as 1000/f.

How to use

Set refractive index and the two surface radii (mm). The calculator updates focal length and optical power automatically.

Example

n=1.5, R₁=100 mm, R₂=-100 mm → symmetric lens; compute f and D using the tool.

FAQs

Q: Can radii be zero? A: No — radii must be non-zero. Q: Units? A: Radii in mm give f in mm and power in diopters (1000/f).