How to use the lens f-number and light gathering calculator
Find the f-number of a lens and how much light it gathers relative to an f/1 lens. Enter focal length, aperture diameter in the fields above; the f-number, light gathering relative to f/1 are computed instantly and plotted on the chart so you can see how the answer changes.
Common searches this calculator answers include “f-number calculator”, “aperture stop”, “how much light lens lets in”, “calculate lens f-number and light gathering”, “find f-number”.
Formula
Variables
- f — Focal length (mm)
- D — Aperture diameter (mm)
Worked example
With focal length = 50 mm, aperture diameter = 25 mm, the calculator gives f-number ≈ 2; light gathering relative to f/1 ≈ 0.25 ×.
Frequently asked questions
What is the lens f-number and light gathering calculator used for?
Find the f-number of a lens and how much light it gathers relative to an f/1 lens. It helps with questions such as “f-number calculator”, “aperture stop”, “how much light lens lets in”.
What is the formula for the lens f-number and light gathering calculation?
f-number = f / D. Light gathering relative to f/1 (×) = 1 / ([f-number] × [f-number]). Here f is focal length (mm), D is aperture diameter (mm).
What inputs do I need for the lens f-number and light gathering calculator?
You need focal length in mm, aperture diameter in mm. Each field is pre-filled with a typical example so you can see how the result responds as you edit the numbers.
How do I use the lens f-number and light gathering calculator?
Type values into the input fields. Results and the chart update instantly. Use Reset to restore the example values, Copy results to paste them elsewhere, or Share to copy a link that preserves your inputs.
Is the lens f-number and light gathering result exact?
Calculations use standard formulas and double-precision arithmetic, then display about seven significant figures. Real-world measurements carry uncertainty, so treat results as estimates unless your inputs are exact.