How to Calculate Focal Length without a Focal Length Calculator?

Can you calculate the focal Length without a focal length calculator? And why should you calculate the focal length of a lens in the first place? Before it was an open question for me how to calculate the focal length of a lens when this is not given. After some intensive research and using an online focal length calculator, I found out there are several ways to do so. When I inherited a cool vintage camera, to my surprise, the focal length was not mentioned on the lens. Most of the time the focal length is printed on the lens. But when I found out the focal length describes the distance from the camera sensor to the lens I knew there must be a way to measure this by myself.

How do you find the distance of the focal length?

The focal length of a lens is the distance, in millimeters, between the optical center of the lens and the sensor of the camera. The optical center is also called the Nodal Point. The Nodal Point of the lens can be considered as the point at which the light rays entering the lens, converge. Remember that the focal length is determined when the camera is focused on infinity. Nowadays, finding the focal length of a lens is easy, because this is always printed on the lens itself. On vintage lenses though, as I discovered myself, this is not always the case so you have to calculate focal length.


How to use focal length creatively?

In practice, the focal length tells you what the angle of view is with a certain lens. The basic rule is the shorter the focal length (or how lower the number on the lens), the wider the angle of view is. On the other hand, how longer the focal length (or how higher the number on the lens), the narrower the angle of view is. I wrote a separate blog post about focal length equations where I compare different focal lengths. You will find different images for the lens equation. Also, I will explain how you can use different focal lengths creatively and for what situation.

Now we understand exactly what focal length is, I’ll take you through the practical steps to calculate focal length without a focal length calculator.

How to calculate focal length without a focal length calculator?

Calculating the focal length of a lens is a pretty interesting and simple process. You can easily measure this with a tape measure! This method you can only perform with converging lenses and not with diverging lenses.

Converging lenses versus diverging lenses

Converging lenses are lenses that converge the light rays coming toward them, whereas diverging lenses are lenses that diverge the light rays coming toward them. The converging lenses form a real image, whereas diverging lenses form a virtual image. A converging lens is also known as a convex lens and a diverging lens is also known as a concave lens. As far as I know, converging lenses are only used for photography and videography cameras, but correct me in the comments section if I’m wrong!


Necessities to calculate the focal length without a focal length calculator

– Tape measure or ruler

– A dark room with a white wall (other colors are also fine

– A strong lamp to create some backlight

Steps to measure and calculate focal length

1. Measure the nodal point of your lens

As you know by now the nodal point is the optical center of a lens. To find this, take your aperture ring or blades as a reference. The distance (mm) from this point to the front of the lens is the measurement you need to perform. Note this distance for yourself in mm.

2. Create a dark room

Make the room in which you are dark and shine with a strong lamp on a free wall. What works better for me is a lamp shining directly into the lens.

3. Finding the focal length of a lens

Grab your lens and hold it (with the rear-facing the wall) in front of the wall so that you can see its projection. You will see when you move the lens closer to the wall, or bring it farther away, the projection will become sharper. Maintain the distance where the projection is completely in focus. This is your focal length!

4. How to measure the focal length of a lens

Measure the distance (mm) from the wall to the back of your lens. This distance added to the distance of the nodal point (measured at step 1) is your focal length(mm). You can also measure the distance from the wall to the nodal point of your lens directly to determine the focal length.

I measure here 50 mm between the wall and nodal point* of the lens (*reference diaphragm ring)

How do you calculate the focal length of a lens for a Micro 4/3 sensor?

My results are 50mm. But I used a 25 mm lens which is a lens made for a Micro 4/3 sensor! To compare the focal length with a full-frame camera sensor, I have to convert the distance by 2 which is the crop factor of a micro 4/3 sensor. A 25mm lens on a Micro 4/3 sensor will result in the same image optics as a 50mm lens on a full-frame sensor. The crop factor of an APS-C sensor is 1.6. So you have to keep the sensor size in mind when performing this measurement. Only for a full-frame sensor the measured distance corresponds directly to the focal length of the lens.

What is the focal length formula to calculate focal length?

When a lens is used to form an image of an object, the distance from the object to the lens (u), the distance from the lens to the image (v), and the focal length (f) are related by:

1/f = 1/u + 1/v

The formula of focal length for the measurement described above is: f ≈v

Online Focal length calculators

You can find a few pages online to calculate focal length with the help of focal length calculators. But with these calculators, you can’t calculate the focal length of a lens easily. You can calculate for example the required focal length to take a picture of an object at a given distance. This is to generate an image fitting your sensor size.

You can find this calculator HERE or another one HERE.

Do you want to learn more?

I hope you learned a lot about how to calculate the focal length of a lens.  If you still have any questions don’t hesitate to ask me in the comments section!

Do you need more information about Focal length? Read my other blog!

FOCAL LENGTH COMPARISON – What is focal length

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One Comment

  1. Hi Atiba, how nice to see and read how you make these beautiful pictures and video’s for your blogs ! Keep on going, because we think your work gets more and more breathtaking.

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