The lens formula :

     (Real is positive convention)

The angular magnification, M, of an optical instrument is defined

by   

                    where  β = the angle subtended at the eye by the image

                              α = the angle subtended at the unaided eye by the object

Simple microscope : 

The compound microscope :

    M =  lateral magnification produced by eyepiece X lateral magnification

           produced by objective.

The Astronomical (refracting) Telescope

( Final Image at Infinity ( Normal Adjustment ) )