The f-number is the ratio of focal length to aperture diameter. A 50 mm lens at f/2 has a 25 mm aperture; each full stop (×√2 in f-number) halves the light. The aperture area sets how much light reaches the sensor.
How to use the Aperture Area Calculator
- Focal length
- f-number N
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the focal length and the f-number. The light ratio compares your f-number with f/1.
Inputs explained
| Field | What to enter |
|---|---|
| Focal length | Units: mm, cm, m, in, µm |
| f-number N | A number |
What each result means
- Aperture diameter = [Focal length]÷[f-number N]
- Aperture area = pi×([Focal length]÷[f-number N])^2÷4
- Stops slower than f/1 = 2×log([f-number N], 2)
Example values
How the main result changes when Focal length changes and everything else stays at the example values above:
| Focal length (mm) | Aperture area (mm²) |
|---|---|
| 25 | 62.6115 |
| 37.5 | 140.8758 |
| 50 | 250.4458 |
| 62.5 | 391.3216 |
| 75 | 563.5031 |
| 100 | 1,001.7834 |
Formula
D = f / N, A = π·D²/4
Worked example
Inputs — Focal length: 50 mm; f-number N: 2.8.
Results — Aperture diameter = 17.8571 mm; Aperture area = 250.4458 mm²; Stops slower than f/1 = 2.971 stops.
Key terms
- f-number
- Focal length divided by aperture diameter.
Frequently asked questions
What does f/2.8 mean?
The aperture diameter is the focal length divided by 2.8.
How much more light does f/2 let in than f/2.8?
About twice as much (one stop).
Does a larger f-number mean a smaller hole?
Yes.
Why do telephoto lenses have huge front elements?
A long focal length needs a large diameter for a small f-number.