In many optical instruments, the light passes through two or more lenses. For example, the simplest optical schemes, such as a telescope or a microscope, include two thin lenses - the objective and the eyepiece (the ocular). The image of the object produced by the first lens becomes, in its turn, the object (real or virtual) for the second lens, which produces the second image of the original object. This second image can also be real or virtual. Thus, the system of two lenses can be expressed by using the thin lens equation twice:

1/s+1/s'=1/f (2)

The distance s2 from the first image to the second lens is equal to l-s'_{1} where l is the distance between the two lenses. The value of s'_{2} found from the equation determines the position of the second image and its type (s'_{2} > 0 for the real image and s'_{2} < 0 for the virtual one).

If the object and both of its images are within a finite distance from the lenses, one can introduce the lateral magnification formula for the lenses

m_{1}=s'_{1}/s_{1}, m_{2}=- s'_{2}/s_{2}

The total lateral magnification of the system of two lenses is equal to the product of lateral magnifications of the lenses.

If the object or its images are infinitely far from the lenses, the lateral magnification provided by the system of two lenses loses relevance.

A fine example in case is a telescope, where both the object and its second image are infinitely far from the lenses.