The experimental process
On the test scene : After setting-up the experiment simply take a picture with the fisheye lens. You can set the camera in any mode which shall give you the maximum Horizontal Field Of View. See this picture as an example.
On your computer : transform this circular fisheye image into an equi-rectangular picture. You may use for this the Helmut Dersch's PTool. It's a plug-in for photoshop (or Graphic Converter on Mac). You may alternatively use PTPiker. You shall then get something like this picture.
Observe carefully a nail on the screen of your computer. It hides some details of the background. Try to identify and remember the missing part.
Go back to the test scene and with a pencil (and possibly with the help of a Square) mark that point on the base of the vertical pattern (on the paper) which matches the identified missing part.
Draw the line which goes from this point through the "hole" made by the nail on the paper. This straight line shall cross the Optical Axis trace on the paper sheet at... the Nodal Point which correspond to the specific ß aspect angle of this specific nail.
Repeat the process for each nail. Notice that the smaller the ß angle (for the nails at the center of the test scene), the more difficult it is to accurately define the intersection.
Next : Test results