Why should you calculate focal length? And when you should, how to calculate focal length? Before this was an open question for me. Now I even searched and used a focal length calculator, found on the world wide web. Lately, I inherited a really cool vintage camera and to my surprise, the focal length was not mentioned on the lens! As you can read in my other blog Focal length comparison – What is focal length, most of the time the focal length is printed on the lens. Just a short reminder; the focal length is the distance from your camera sensor to your lens. So there was my answer to my question.
You have to calculate focal length when it’s not printed on your lens (most with vintage cameras, lenses) and you really want to know it to understand the camera more and how to use it creatively. Luckily I’ve found some great lens focal length calculators online and with some research I was able to calculate the focal length of the vintage camera. I assume you might be in the same kind of situation because you are here. Don’t look further because now I know how to calculate focal length and in this blog I will explain the steps to calculate focal length!
Before I will take you through the practical steps of the focal length calculation, I’ll explain what focal length is. Feel free to skip this explanation if you already know all about focal length.
How to find 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. Take in mind the focal length is be determined when the camera is focused on infinity. Nowadays, finding the focal length of a lens is easy, because this is always printed on modern lenses. On vintages 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, focal length tells you what the angle of view is with a certain lens. The basic rule is; how 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 about focal length equation 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 in what situation. Please find this blog here: Focal length comparison – What is focal length
Now we understand exactly what focal length is, I’ll take you through the practical steps to calculate focal length!
25mm (50mm for Full Frame Sensor)
This focal length is said to be identical to the human eye. This image is what you see with your eyes, in terms of distance.
100 (200 Full Frame Sensor) mm
This focal length compressing the image a lot and see how close the background is compared to the image shot with 25mm.
How to calculate focal length of a lens
Calculating focal length of a lens is a pretty interesting and simple process to do. 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 which converge the light rays coming towards them, whereas diverging lenses are lenses which diverge the light rays coming towards them. The converging lenses form a real image, whereas diverging lenses form a virtual image. A converging lens is also known as convex lens and a diverging lens is also known as concave lenses. 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 work out focal length
The following steps explain how to work out focal length:
– Tape measure or ruler
– A dark room with a white wall (other colors are also fine
– A strong lamp to create some backlight
Step 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.
Step 2: Create a dark room
Make the room in which you are dark and shine with a strong lamp on a free wall. What worked better for me is a lamp shining directly into the lens.
Step 3: Finding 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, sounds logical right?!
Step 4: How to measure the focal length of a lens
Measure the distance (mm) from the wall till the back of your lens. This distance added to the one of the nodal point, which you 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 diafragma ring)
How do you determine the focal length of a lens for a Micro 4/3 sensor?
My results are 50mm. But the lens which I used for it was a 25mm..did I do something wrong? No, because its a lens made for a Micro 4/3 sensor! To compare the focal length with a full frame camera sensor, you have to convert the number by 2 (crop factor). So a 25mm lens on a Micro 4/3 sensor will result in the same image as a 50mm lens on a full frame sensor. The crop factor of an APS-C sensor is 1.6. So you have to take the sensor size in mind when performing this measurement. Only for a full frame sensor, you can directly relate the measured distance to the focal length of the lens.
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
Focal length calculators I’ve found online
You can find a few pages online to calculate focal length with focal length calculators. But with these calculators you can’t really calculate focal length of a lens in an easy way. You can calculate for example the required focal length to take a picture of an object at a given distance. This in order to generate an image fitting on your sensor size.
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!