Enter the distance of the object and the distance of the image into the calculator to determine the focal length.

Thin Lens Equation

The following formula described the thin lens equation.

1/do + 1/di = 1/f

  • Where do is the distance to the object
  • di is the distance to the image
  • f is the focal length

Thin Lens Definition

A thin lens is defined as a lens in which the focal length is the same on either side of the lens. Parallel lines of light passing through one side of the lens will converge at the focal point of the opposite side.

Example Problem

How to calculate the focal length of thin lenses?

  1. First, determine the distance from the object to the center of the lens.

    For this example, the object is found at 50ft from the center of the lens.

  2. Next, determine the distance from the center of the lens to the projected image on the opposite side of the lens.

    For this problem, the projected image is found to be at 45 ft from the lens.

  3. Finally, using the formula 1/do + 1/di = 1/f, calculate the focal length.

    Enter the information into the equation, we find the focal length to be 1/50 + 1/45 = 1/f , f = 23.68 ft.

thin lens calculator