For a telescope in normal adjustment, the magnifying power is simply the ratio of focal lengths (f_o/f_e). When the final image is formed at the least distance of distinct vision, the eyepiece acts as a simple microscope, and the magnification increases by a factor of (1 + f_e/D). This is derived from the principle that for a simple microscope, when the image is at infinity, m = D/f, and when the image is at distance D, m = 1 + D/f. The ratio gives the additional magnification factor.