In a telephoto lens, the magnification of the subject is achieved through a combination of convex and concave lenses. In its simplest form, a convex lens on the outside converges light.

Concave lens: This lens is thinner at the centre and thick at the two sides. ... Magnification (m) = height of image (h 2) /height of object (h 1) Another formula in terms of distance; ...

A lens can be converging (convex) or diverging (concave). What are converging ... The magnification of lens is defined as the ratio close of the height of the image to the height of the object.