Finding the Nodal Point of a Lens
What is the Nodal Point and Why is it Important?
The nodal point of a lens is the point inside a lens where light paths cross before being focused onto a film plane. When taking the pictures for a QTVR Pano movie, you want to rotate the camera around a line that runs through (or very close to) the nodal point of the camera lens. Failing to do this may result in problems due to parallax during the stitching process. Parallax is the apparent shifting of foreground objects relative to background objects when you change your point of view position.
One way to experience parallax is to hold out your hand ,and with one eye closed, place your finger over an object you see in the distance. When you close one eye and open the other, your finger apparently "jumps" relative to the distant object. This is parallax due to your point of view shifting a few inches, from one eye to the other.
Parallax is a problem in QTVR Panoramas because they are generated from several overlapping pictures that have been "stitched" together to make one panoramic picture. If an object which occurs in the overlap of two adjacent shots is near the lens, and the nodal point of the lens moves between the shots, then the apparent position of nearby object will shift relative to the background. When the software tries to stitch the two shots together, a blurring or ghosting will occur on the edges nearby object as it blends the object's shift with the background.
Parallax is more of a problem when there are objects within a few feet of your camera when taking a panorama. In that case you should try to get your pan head's line of rotation as close to the nodal point as possible. If you are taking a pano of a landscape where everything is far away, you can be off the nodal point considerably with no ill effects.
Determining the Nodal Point of Your Lens
By "finding the nodal point of lens," I mean determining how you should mount your camera on your pan head so that you know the pan head's line of rotation runs through the camera's nodal point. Once you find this mounting position, you can mark it or record it so you can quickly find it when you set up for a QTVR Pano shoot. The Kaidan Kiwi+ pan head scales that are designed for that purpose. Once you have determined the nodal point for a given lens, record the scale readings for future use. This example shows how I found the nodal point of a Kodak DC50 camera on a Kaidan Kiwi+ pan head.
Finding the Nodal Position Across the Diameter of the Lens
- There are two orientations that correspond to 2 scale readings that you will need to find. The first is the location of the nodal point across the diameter of the lens. Begin by setting up your tripod and mounting the camera on the pan head as you would do for a Pano shoot. Be sure the camera is level. Find some nearby object to judge parallax with. Here, I set up near a light post, so that I can judge its hard edge with a feature in the background.
- Move the vertical bracket holding the camera over so that the center of the lens diameter is directly over the line of rotation of the pan head. You can use a plumb line (e.g. a key tied to a piece of string, dangled in front of the lens) to help you find the position accurately. Once you find the position, mark the setting on your pan head or record the reading on the scale.
- Now you need to slide the camera itself over the rotation point to find the other orientation of the nodal point, inside the lens. Start off by placing the camera so that the line of rotation runs through the front surface of your lens. The nodal point will be behind this position. Set your tripod so that you can rotate the camera and have the edge of your nearby object show up on two different sides of a pair of shots.
- If you have through-the-lens (TTF) focusing or a LCD display on your camera, you can use that to judge when you have found the nodal point. The DC50 camera has a viewfinder that is not TTF. The viewfinder lens cannot be used to determine the nodal point of the picture-taking lens, so I had to take a pair of shots, move the camera a few ticks of the scale and take another pair. I took several pairs of shots, moving the camera back about 5 mm each time, until I felt I had passed the nodal point of the lens. I carefully recorded the scale readings of each of the pairs of shots. If I had a TTF view finder or LCD display, I could have recorded the scale reading immediately.
What a pair of pictures look like when you are off the nodal point. (somewhat exagerated)
The pair of shots at the scale reading of 55. The nodal point has been found!
- After taking the shots, I downloaded them to my computer, and examined them carefully, looking for the pair that showed the least amount of shift. That pair will be the closest to the nodal point. If you think you need to get closer, you can take another set of shots, moving the camera a smaller increment between them. In my case, I did not think I could improve on the setting I found using 5 mm increments.
- Be sure to record the settings you have found, so that you can use them to quickly set up for each of your Pano shoots. If you use a different camera or lens, of course, you'll have to go through this same process to find those scale readings for that nodal point. On zoom lens, the nodal point moves as you zoom. I always shoot Panos with the DC50 zoomed to maximum wide angle.
Here is the correct setup of the Kodak DC50 on a Kiwi+ tripod, with scale readings for the nodal point.
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