Ronja Twibright Labs

How do I measure focal length of a lens?

Put the lens parallel with a wall or white flat surface and focus distant light source (streetlam), horizon or lamp in your room on the wall until you get sharp image. The lamp must be near optical axis of the lens. a is distance to lamp, streetlamp, or horizon. a' is distance to wall. Calculate f from a and a'.

For more info about focal length see Focal length on Wikipedia

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