How to stretch long legs with SLR

I don't know how long legs you need, but basically the second floor has already answered. I only add some figures to illustrate; The focal length is 50mm or less (135 camera conversion), and it is shot back below the waist. The wider the wide angle, the longer the legs and the lower the camera position. Pay attention to the position of the head, don't be too far from the center or pat the head, otherwise MM's face will be widened. This is all done by using lens distortion.