Magnification is measured by comparing the sizes of the image and the object (linear magnification), or the sizes of two images obtained in different conditions (visual magnification).
Linear magnification M1 is given by the ratio between the size of the image (real or virtual) hi and that of the object ho:
Ml = hi/ho
Magnification = 1 means that the image is the same size as the object.
Magnification greater than 1 means that the image is bigger than the object.
Magnification smaller than 1 means that the image is smaller than the object (it is still called magnification, even though it is actually a reduction!).
Images (both real and virtual) produced by optical instruments are often inverted. In that case, by convention, a minus sign (-) is placed before the magnification value, denoting negative magnification.
Visual magnification Mv is determined by comparing the observations made with an instrument and with the naked eye. If hit is the size of the image on the retina when using, for example, a telescope and hin the size when using the naked eye, the visual magnification is given by:
Mv = hit/hin