Enter the lends focal length and the aperture circle diameter into the calculator to determine the f-stop. This calculator can also determine the focal length or aperture diameter given the other variables are known.

F-Stop Formula

The following formula is used to calculate and F-stop.

Fstop = FL / AD
  • Where F-stop is the correct camera setting for an f-number
  • FL is the focal length
  • AD is the aperture diameter

To calculate an f-stop, divide the focal length by the aperture diameter.

F-Stop Definition

What is an f-stop? An f-stop is a ratio of the focal length of a lends to its aperture area. F-stop, also commonly referred to as f-number, is usually displayed on a camera when you change the size of the lens.

Example Problem

How to calculate F-stop?

  1. First, determine the focal length.

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

  2. Next, determine the aperture diameter.

    For this problem, the aperture diameter is found to be 5mm.

  3. Finally, calculate the f-stop.

    Using the formula above, the f-stop can be calculated. F = FL/AD = 10/5 = 2.

About F-Stop

What does f-stop affect? Different f-stops will affect the focus, sharpness, and the quality of the image all around.

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 the focal length to aperture diameter while a shutter speed is a simple measure of how long a shutter is open for a picture.