How to Measure the Focal Length of a Lens

You can determine the focal length of a lens by using a piece of paper.

The "focal length" of a lens is best used as a reference for the type of lens that is being a used. A lens that has a 24mm focal length is known to be a wide angle lens. A 200mm lens conversely is known as a telephoto lens. Photographers are lucky, since the focal length of each lens is clearly indicated on the lens barrel. If you have an individual lens and you want to determine the focal length for the lens, you can do this at home.

Things You'll Need

• Lens
• Paper

Instructions

1. Find a scene with objects that are at a medium distance. These can be buildings, landscapes or any other object that is neither too close to you nor too far away.

2. Hold the lens up in one hand so a scene is in front of the lens, and hold a piece of paper behind the lens so the scene can pass through the lens onto the paper.

3. Adjust the paper by moving it closer or further away until the scene behind the lens is reflected wholly onto the paper, and is in focus.

4. Measure the distance between the paper and front of lens; that distance is now the focal length for that lens.

References

• Giangrandi: Lens Focal Length Calculator
• Physics: Finding Focus Length of a Convex Lens
• Bob Atkins: Measuring Focal Lenght