f = 1 / (1/a + 1/b)

Measuring focal length

Unless you already know the focal length of your lens, you should measure it. You'll need a piece of paper, a ruler, and a distant light source such as a lamp across the room (do NOT use the sun for this).

Project the light from the lamp through the lens and onto the paper. Adjust the distance between the lens and the paper until the image is at its sharpest. Now just measure the distance from the lens to the image on the paper. This is the focal length of the lens, or close to it. Ideally, the light source should be an infinite distance away, but I find that a light across the room gives me a pretty good approximation. (If you insist on precision, then also measure the distance from the object to the lens and use the formula shown)

Depending on how we address the mechanics of the lens, focal lengths in the range of 60 - 200 mm are good candidates for DIY camera lenses.

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