A Simple Explanation of F-Stop Numbers

Even if you have a good command of using f-stop numbers and properly exposing photographs, you might not understand the math behind why f-stop numbers are what they are. Here’s a simple (albeit math-filled) explanation by Dylan Bennett of what f-stop is, including a simple trick you can use to memorize the f-stop scale.

  • Ndt

    Me want peanuts.

  • Miguel Moragues

    Great video, thanks!  I have a doubt. How come there are lenses with F/1.2 or F/1.8 apertures?

  • Randy S. Little

    There are lens with f0.9

  • PapaEcho

    It’s just an in-between ratio, probably chosen for manufacturability and cost.

  • Ed

    Yep. In-between ratio, i.e. they don’t double/divide in two the diameter and, consequently, the amount of light passing through the aperture. For instance f/1.6 and f/1.8 are thirds of a stop between f/1.4 and f/2; they let in roughly 1.3x and 1.6x the light that goes through f/1.4. I am sure the maths are deeper, but that’s a practical understanding.

  • Jim Greif

    An excellent video.
    I am attempting to explain digital sensors and how they accept light for a niece. She is only 10 years old but very bright, and she wants to do a school project on this topic. Do you have a video on sensors and how they work? Thank you, Jim

  • Richard

    If I stop my brain down to f/44 I start to get the math but not much else gets in…

  • Gekgjnejkfn

    That alternate doubling trick is handy, finally a way of easily working out that scale. Not that I ever really needed to do that…

  • Slide rule user

    “math-filled” ? “Hard to remember” f-stop scale? Diameter and area (!!!) both expressed in millimeters??? OMG…

  • Guest123

    if the opening is 20mm how come the area is not 1256 ( a = pi*r^2 = 3.14* 400 ) but is 314, just a quarter? 


    the diameter is 20 not the radius

  • Matt

    I’ve written a similar explanation at It’s worth running the traps once on the math to understand how it works.

  • Creek Crab

    Great video! Cleared up my concepts for sure. To the point, graphical and best of all it was simple.