Thin Lens Equation Calculator

By Dominik Czernia, PhD candidate
Last updated: Dec 02, 2020

Thin lens equation calculator has been prepared to help you to analyze optical properties of the simple lens. Keep reading to learn about the thin lens equation and understand how a lens can magnify the image of an object. Everything is about light, so make sure to check out the principles of the light refraction too!

Thin lens equation

If we place the object near the lens, we will get its image somewhere. The position, orientation, and size of this image depend on two things: the focal length of the lens (which is specific for the particular lens) and the position of the original object. We can predict what we will see with the following thin lens equation:

1/x + 1/y = 1/f


  • x is the distance between the object and the center of the lens,
  • y is the distance between the image and the center of the lens,
  • f is the focal length of the lens expressed in length units.

There are two basic types of lenses. We can distinguish converging lenses which have focal length f > 0 and diverging lenses for which focal length f < 0. It should also be noted that when image distance is positive y > 0, then the image appears on the other side of the lens and we call it real image. On the other hand when y < 0 then the image appears on the same side of the lens as the object, and we call it virtual image.

Magnification lens equation

In the advanced mode, you can compute the magnification of created image too. It can be easily estimated if we know the distance of object x and the distance of image y:

M = |y|/x

Remember that magnification must always be a positive number. That's why we have taken the absolute value of y which generally may be both positive and negative.

Images in the converging lens

Let us consider five different situations for a converging lens (f > 0). You can check it with our thin lens equation calculator!

  • for x > 2f image is real (y > 0) and diminished (M < 1);
  • for x = 2f image is real (y > 0) and of the same size as object (M = 1);
  • for 2f > x > f image is real (y > 0) and magnified (M > 1);
  • for x = f image doesn't appears (y -> Infinity);
  • for x < f image is virtual (y < 0) and magnified (M > 1).

We encourage you to check similar cases for the diverging lens which have negative focal length f < 0 with our calculator!

Dominik Czernia, PhD candidate
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