The image recorded by a photographic film or image sensor is always a real image and is usually inverted. When measuring the height of an inverted image using the cartesian sign convention (where the x-axis is the optical axis) the value for "hi" will be negative, and as a result "M" will also be negative. However, the traditional sign convention used in photography is "real is positive, virtual is negative". Therefore, in photography: Object height and distance are always "real" and positive. When the focal length is positive the image's height, distance and magnification are "real" and positive. Only if the focal length is negative, the image's height, distance and magnification are "virtual" and negative. Therefore, the "photographic magnification" formulae are traditionally presented as