The Nodal Point for Panoramas
Panoramas can bring their own unique problems an done of these concerns the 'Nodal Point'. To be more accurate and in particular if some of the subject will be near the camera, then the camera's point of rotation must be under such a position. Without this, when the different images are stitched together, all the subject matter will not line up. As you rotate the camera, a distant and a near object will move in relation to each other.
Rays of light enter a lens, cross over and extend to the point of focus. Where the light paths cross is the nodal point. Well not exactly; the term has been used for decades but technical purists will explain that it is a misunderstanding of the term. The position where rays of light converge is more correctly called the 'entrance pupil' of the lens, but for the purpose of this tutorial we'll use the familiar lingo.
The point will be inside the lens behind the front surface. The camera should rotate at this point otherwise foreground objects and background objects will move in relation to each other as the camera rotates. It's a bit like holding up one finger in front of the face; look at a distant object through each eye alternatively and the finger appears to move from side to side.
To Find the Nodal Point
- The camera must be set on the tripod in two directions; forward and backward, left and right. The panoramic bases made by Kaidan, for example, are ideal for this.
Firstly, the centre of the lens must be over the tripod bush. The camera bush is rarely directly in line with the lens, so the camera needs to be adjusted left and right to achieve this. A plumb-line can help in checking the alignment.
- Having the camera mounted on a macro focusing rail will do the job very well. The centre of the lens will be over the tripod bush. The camera can then be moved backward and forward independently of the tripod attachment.
- Short of having the right gadget, it is not too difficult for anyone handy with tools, some plywood or metal plate and a bit of creativity, to make a home-made alternative.
- Now the entrance pupil itself needs to be found. It will be behind the front surface of the lens, just inside the lens. With a single-lens-reflex camera this is fairly quick. Non-SLRs are mentioned below.
- Set up the tripod 1 metre from a window frame and looking outside to a distant object.
- Set focus to infinity at the distant object.
- Rotate the camera slightly from side to side, while looking through the viewfinder. It is likely that the distant object will move in relation to the side of a window frame vertical. The two objects will either move closer to each other or farther apart, meaning the lens is not rotating on its nodal point. This means there is a parallax error which will make successful stitching virtually impossible.
- Slide the camera back and forward on the rails while rotating it from side to side until you find the position where the distant object does not move in relation to the window frame. As you get closer to the nodal point, the shifting will get less, stop altogether and then reverse as you go past the optimum point. You want the position where there is no shift. This will be the position where there is no parallax error and the camera will be rotating at its nodal point.
- Mark this position on the focusing rails for all panorama set-ups with this lens. Another lens with a different focal length will have a different nodal point.
- If your camera does not have through-the-lens viewing, the viewfinder is of no help here. The procedure is a bit longer, but not too bad if the camera is digital. Set the centre of the lens over the rotation point first by using the left and right adjustment.
- To set the critical position over the rotation point, move the camera backwards and forwards so that the lens' front element is slightly behind the rotation point, judging it by eye.
- Line up on the near and far objects and rotate the camera a little to the left and right taking one picture in each position, producing a pair of images.
Move the camera forward 5mm and take another pair of images, taking notes as you go. Take a series of images in pairs this way.
Download the images on to the computer and compare them. Find the images where there is no movement between the objects and they will have been taken at the nodal point position.
If the results are not quite accurate, take another pair of photographs at finer adjustments. On the camera base, mark the position accurately for future use.
- When using a zoom lens, the nodal point will move with each magnification.
For accuracy, it's best to use a zoom lens at either the longest or widest setting and forget about those in the middle.
- When the camera is to be used vertically in portrait format, getting is good alignment is more tricky. The macro focusing device is not suitable and a DIY gadget is more complicated. The brackets made by Manfrotto, Kaidan and Novoflex are much more justified here.
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Tripod Heads for Panoramas
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