The mirror equation relates the focal length f with the distance of the object from the mirror, do , and the distance of the image from the mirror, di , as the following,

( ho + hi ) / do = hi / f ( ho + hi ) / di = ho / f ( ho + hi ) ? 1 / di + 1 / do = 1 / f

The magnification, m, defined as the ratio of the image size, hi , to the object size, ho , is related to di and do by m = hi / ho = di / do

do and ho are always positive. If hi is positive, the image is erect; if hi is negative, the Image is inverted.