Node (photography)

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As nodes ( engl. Nodal point of node "nodes") or nodal points of an imaging optical system is defined as the points on the optical axis on which the light rays which enter at the same angle to the optical axis in the system, as it also left again, apparently running towards (front nodal point) or from which they apparently start (rear nodal point). If there are media in front of and behind the optical system with the refractive indices for which the optical system was designed (typically n = 1, air), the nodal points coincide with the main points (points of intersection of the main planes with the optical axis).

The pivot point relevant for panoramic photography does not normally coincide with a nodal point, but with the position of the entrance pupil of a lens, which only in a special case (a single thin lens without an additional aperture) coincides with both nodal points (see also The entrance pupil in panoramic photography ).

Nodal point adapter

Pivoting camera on a "nodal point" adapter

A nodal point adapter enables z. B. to rotate a camera or a lens (or both together) around a freely selectable axis (within the mechanical limits of the adapter) - z. B. around the center of the entrance pupil or around one of the two nodal points. With the help of a nodal point adapter and a collimator , the nodal points of an objective can be determined.

Nodal point adapters are mainly used when creating panoramas from single images . For this purpose, the adapter is set so that the axis of rotation coincides with a diameter of the entrance pupil (and not with one of the nodal points and also not with the image plane).

Determination of the nodal point

In order to determine the position of the projection center of a camera-lens combination with a nodal point adapter or a panorama head, on the one hand the height (transverse offset), but also the depth (the actual determination of the nodal point) in relation to the screwing point of the camera must be determined. There are different approaches to this. First, the height should be determined. It is suitable for this if this is measured directly on the bayonet of the camera, or at the front of the lens if this is permanently installed. Because an exact center height is often missing, this must be calculated from two other measured heights. If the camera is on a table, measure the height from the edge of the table to the lower limit of the bayonet and the height to the upper limit. The arithmetic mean is then formed from these two heights in order to calculate the center height, which can then be set directly with most nodal point adapters or panorama heads.

The determination of the depth value is only possible experimentally and therefore requires a little patience. Systematic testing is possible with different test tracks, but basically a near and a distant object is required. To make parallax errors clear, the objects should be as far apart as possible. The position of the nodal point depends on the set focal length. Zoom lenses must therefore be set to the desired focal length. With most lenses, the nodal point also shifts with the focus setting. The autofocus function must be switched off here. Since both objects must be clearly visible, you need a large depth of field and therefore close the aperture as far as possible (e.g. to 8 or more). The lower limit of the depth of field (see aperture ring) can serve as a guide value for the distance for the close object. The distant object should be at least twice as far away as the highest specified focus value before infinity. Both objects should be made in such a way that they can be aligned. A classic setup would be, for example, if you put the camera with tripod and nodal point adapter in front of a window and attach a plumb line to the window frame. Alternatively, you could stick a writable adhesive strip to the window pane and draw a line on it. This then corresponds to the nearby object. As a distant object one could choose, for example, the gable top of a building, a spire or a vertical mast. It is advantageous if the camera is switched to live view mode, if this is possible. Now move and turn the tripod so that both objects are close to the left edge of the picture and are in line with one another. Then you turn the nodal point adapter or let motorized panorama heads move so that both objects move from the left to the right edge of the image. If the depth setting is correct, the position of the two objects must not change. So they should always be on top of each other, no matter how the camera is rotated. If the distant object is shown to the left of the near object, the depth setting must be increased (shifting the camera backwards). Otherwise, if the distant object is shown to the right of the near object, the setting value for the depth must be reduced (shift the camera forwards). The determined setting values ​​can be used again and again with the same combination of camera, lens and focal length.

See also

credentials

  1. ↑ It can be assumed that in many publications in connection with the creation of panoramas the position of the entrance pupil is confused with the nodal point

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