Focal Length Calculator

Created by Łucja Zaborowska, MD, PhD candidate
Reviewed by Steven Wooding
Last updated: Apr 22, 2022

The focal length calculator is a simple tool that facilitates the process of computing the magnification, focal length, and the angle of view.

Taking a picture of an object from a distance might be challenging - we'll help you find the right focal length to create an image that perfectly fits your cameras' sensor size. 📷

In the article below, we'll teach you how to find the focal length, discover the lens equation, and talk about a few essential principles of photography and the lens choice.

What's the focal length?

Focal length is one of the primary values of a photographic lens. Manufacturers usually give it in millimeters (mm).

Focal length describes the distance between the rear principal point and the sensor - in other words, it's the space starting from the center of the lens, to the point where the light rays converge in the focal point (to form a sharp picture on a surface of a digital sensor, or 35mm film).

Biconvex lens-behaviour of light rays from focal point
Biconvex lens-behavior of light rays from the focal point.
Credit: Kvr.lohith, CC BY-SA 4.0, via Wikimedia Commons.

💡 Focal length can be determined only when the lens is focused at infinity

🖼️ Thanks to the focal length, we're able to calculate the angle of view - this variable informs us of the amount of the scene that will be captured. The wider the angle, the more of the scene can be transferred to a sensor, and be visible on the photograph. Thanks to the angle of view option, we don't really need an additional field of view calculator - we got everything ready!

Lens' Focal length principles

🔍 Thanks to our lens focal length calculator, you may also find the magnification - it allows us to measure how the size of an object changes when transferred to photography.

Still hungry for knowledge? Try our other lens calculators:

Or learn how the concept of focal length is fundamental when we need to calculate parameters like the camera field of view!

How to use the focal length calculator?

Our lens equation calculator has a straightforward structure; fill in at least three fields in order to acquire results.

  • Typical image sizes:

    • 3.6, 4.8, 5.8, 6.4, 8.8, 12.8 mm,
    • or 1/4, 1/3, 1/2.5, 1/2, 2/3, 1 inch.

    (Hey, if you're still not familiar with different units, try our length conversion tool 😉)

  • Object distance is measured from the lens's front principal plane to the object itself.

💡 Remember, our calculators work both ways. Your result can simply become another query!

The lens that makes objects appear small will have a small magnification - on the other hand, the lens that magnifies the picture will have a big magnification.

How to calculate focal length?

Finding focal length is an easy and very much needed ability; follow our simple guide to find all the details of these calculations!

  1. The typical focal length formula looks as follows:

    1/Focal length = 1/Image distance + 1/Object distance,


    • Image distance and Object distance are given in mm.
  2. And here's the transformed equation that we use:

    Focal length = (Object distance / ((1 / Magnification) + 1)) * 1000,


    • Object distance is given in mm; and
    • Magnification does not have a unit.
  3. In order to copy the lens magnification calculator, you'll need the following equation (take a closer look - it may also serve as an image distance formula!):

    Magnification = Image size / Object size = -(Image distance / Object Distance),


    • Object size - is the real size of an object, given in mm; and
    • Image size - is the size of the camera's digital sensor or 35mm film, given in mm.
  4. To calculate the angle of view, you need to use the most complex equation of them all:

    Angle of view = (180/π) * 2 * aTan(Image size / (2 * Focal length * (Magnification + 1))),


    • aTan(x) means the arc tangent , described as the inverse tangent of a function x (in radians).
Łucja Zaborowska, MD, PhD candidate
Object size
Image size
Object distance
Focal length
Angle of view
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