The nodal point of a lens, sometimes known as the entrance pupil or principal point, must be located as accurately as possible otherwise virtual realirty (VR) images will exhibit parallax distortion and be impossible to stitch together. Only when the nodal point is located directly on the axis of rotation of the tripod head will the images be free of parallax distortion. Parallax distortion is the apparent change in the direction of an object caused by a small change of viewpoint.
Several methods can be used to locate the nodal point for a particular camera and lens combination. However, the simplest approach is to set up the camera and lens on a panoramic tripod head and position two tall and thin alignment markers, such as pencils, in front of the lens. The nearer marker should be close to the lens - perhaps about one metre distant, and the further one should be at least a few metres away. The camera is best positioned slightly above the level of the alignment markers so that the more distant marker can be seen above and beyond the closer marker as in the picture above. Initially, it is important to position the two markers and the camera so that they appear precisely aligned and central in the viewfinder (see the central image).
Having set up the camera and lens, and aligned the two markers, rotate the camera to one side and observe any change due to parallax - ie the two markers appearing to move out of line as they approach the edge of the frame (see the left and right hand images). Rotation of the camera in the other direction should produce apparent misalignment in the opposite direction (assuming the nodal point is not yet correctly set). If misalignment due to parallax is observed, move the camera forward or back on the slider of the panoramic head and repeat the observation process. Continue with such adjustments until the alignment of the two markers remains unchanged as the camera is rotated from side to side. The position that reveals no misalignment of the markers as the camera is rotated should be carefully marked on the panoramic head as the nodal point for the particular camera and lens combination.