Calculate f-number, aperture diameter, or focal length with real unit conversion, aperture area, nearest full stop, and exposure-light comparison.

Use focal length and effective entrance-pupil diameter.

Geometric entrance-pupil estimate. Exposure comparison assumes equal shutter speed, scene and lens transmission; close-focus effects are excluded.

Effective aperture as viewed through the front of the lens.

Compare apertures

Blank omits the exposure comparison.

F-Stop Formula

The following formula is used to calculate an f-stop (f-number).

N = f / D
  • Where N is the f-stop (f-number)
  • f is the focal length
  • D is the entrance pupil diameter (often approximated as the aperture diameter)

To calculate an f-stop, divide the focal length by the entrance pupil (aperture) diameter.

F-Stop Definition

What is an f-stop? An f-stop (f-number) is the ratio of a lens’s focal length to the diameter of its entrance pupil (often approximated as the aperture diameter). It is usually displayed on a camera or lens when you change the aperture setting.

Example Problem

How to calculate F-stop?

  1. First, determine the focal length.

    For this example, the focal length of the camera is 10 mm.

  2. Next, determine the aperture diameter.

    For this problem, the aperture (entrance pupil) diameter is found to be 5 mm.

  3. Finally, calculate the f-stop.

    Using the formula above, the f-stop can be calculated. N = f/D = 10/5 = 2.

About F-Stop

What does f-stop affect? Different f-stops affect exposure (how much light reaches the sensor/film) and depth of field; very small apertures (high f-numbers) can also reduce sharpness due to diffraction.

Is f-stop the same as shutter speed? An f-stop and shutter speed are not the same. F-stop refers to the ratio of focal length to entrance pupil (aperture) diameter, while shutter speed is a measure of how long the shutter is open (typically in seconds or fractions of a second).