What this calculates
A camera’s f-number expresses focal length relative to the diameter of the entrance pupil (the effective aperture opening). This calculator finds the physical diameter and area of that opening, which determines how much light reaches the sensor.
The formula
The aperture diameter is D = f / N, where f is focal length and N is the f-number. The area is then A = π × (D/2)². Since area scales with the square of diameter, each full f-stop step (like f/2.8 to f/4) roughly halves the light-gathering area.
This calculator is for general photography and optics reference.
Last reviewed August 2026