What is Parallax?
Parallax causes adjacent pictures for a panorama to differ in ways that prevents them from being stitched together perfectly. It can cause ghosting, blurring, or even prevent stitching software from being able to work out where to position the pictures to be able to stitch them together.
It's really easy to see the effect of parallax: Hold up your index finger in front of you. Close one eye and line up your finger with something further away such as a door, piece of furniture, window, whatever. Now without moving your finger, rotate your head from left to right - your finger will seem to move slightly as you turn your head. Voilà! you are seeing parallax.
Exactly the same happens when you shoot pictures with your camera without using a panoramic head. The amount of parallax shift depends on how far off-centre the camera lens is - I have prepared a short video of parallax as seen by the camera. To completely elimate the parallax effect, the camera needs to rotate around the no-parallax-point of the lens.
|Example shots taken without panoramic head showing parallax|
In the above pictures note that it looks as if the trees in the background have moved slightly between the shots. As a result when the two pictures are stitched together either the tree in the foreground won't align, or the ones in the background won't. That will mean there will be blurring along the seam.
"No-parallax-point" vs "nodal point"?
The majority of websites (including this one until recently) referred to the no-parallax-point as the nodal point. There has been plenty of discussion in various Internet forums and there is a very good article about the no-parallax-point on the panotools wiki discussing whether this is correct. Rik Littlefield has written a more technical paper about the no-parallax-point. Panoguide has adopted the proposed terminology "no-parallax-point" as this accurately describes the point we are interested in.
How to avoid parallax
If everything in the scene is approximately the same distance from the lens, or very far away, the parallax effect will be elminated or at least minimized. This is why hand-held panoramas from a mountain top tend to work quite well - everything is far away from the camera, and because the distances are very large compared to the tiny distance between the nodal point and the point of rotation, parallax is negligible.
Wide angle lenses can help reduce parallax effects because a wide angle lens accentuates perspective, making everything seem further away. However wide angle lenses also have a larger field of view so there is a greater chance that things in the foreground (including the ground itself) are captured and then cause problems. Providing there are no foreground objects juxtaposed against far away objects parallax will be minimized and you may be able to successfully post-edit any glitches in the stitch to achieve a good image. Conversely long focal length lenses compress persective, exaggerating parallax effects.
The only way to eliminate parallax entirely is to use a panoramic head, which allows you to accurately position your camera so that when you take the pictures you are rotating the camera around the nodal point of the lens.