As mentioned earlier, magnification by a lens is defined as the ratio of size of the image to that of the object. Most of the time, we consider a point object or a line object whose distance from the lens is fixed. In general, our object may have some thickness and the distance of two end points of object may be different from the lens as shown in figure. For simplicity we have drawn a tilted arrow AB. The image of this arrow is termed as A'B'. The horizontal separation between the end points of object arrow is O_L, whereas the vertical separation is O_T. The corresponding separations

for image I_L and I_T. We define the transverse and longitudinal magnifications as

                        Transverse/Lateral Magnification   

                         Longitudinal Magnification    

When the object distance, image distance and focal lengths are much larger compared to O_L, the lateral magnification tends to unity.