Lens Calculator
Describe your beam and where you need it — the tool recommends the right lens.
Given an input beam diameter, target output beam size or focal spot, working distance, and wavelength, this tool selects the appropriate lens type and focal length using the thin lens equation (1/f = 1/do + 1/di) and Gaussian beam propagation. It handles focusing, collimation, and beam expansion scenarios, and estimates the diffraction-limited spot diameter at the focal plane. The ray diagram updates in real time as inputs change. Use this tool when setting up a focusing objective, collimating a diode laser, or designing a beam expansion telescope.
Your Beam
What You Need
Lens Parameters
Best for collimated-to-focus. Orient curved side toward collimated beam.
Recommended Lens
100 mm FL · 1/2" ⌀ · f/7.9 · N-BK7 · Plano-Convex
Calculated Results
Minimum Lens Diameter
85% clear aperture → next standard size: 1/2" (12.7 mm)
Radius of Curvature
f-number
Focused Spot Diameter (1/e²)
Spherical Aberration (est.)
Within diffraction limit
Airy Disk Diameter
First zero of diffraction pattern (2.44 λ f/#)
Depth of Focus (2× Rayleigh range)
Back Focal Length (est.)
Based on ~2.0 mm estimated center thickness
Refractive Index — N-BK7
At λ = 632.8 nm (Sellmeier equation)
Abridged Optics — Lens Calculator v1.0Gaussian beam calculations assume TEM₀₀ mode