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: “Compare focal lengths – What is focal length”.
Now we understand exactly what focal length is, I’ll take you through the practical steps to calculate focal length!
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