Lensmaker’s Formula

With the proper sign conventions (see text), the same formula may be used to analyze images formed by thin lenses and mirrors:

where s_o is the object distance, s_i is the image distance, and f is the focal length. Negative image distances correspond to virtual images, and positive image distances correspond to real images.

The accuracy of this equation is subject to the validity of the paraxial approximation which requires that the focal length be much greater that the diameter of the optic.