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NN3 & NN3 MKII Complete Manual

12. Finding the Entrance Pupil of a Lens

What is the Nodal Point or Entrance Pupil and why is it important?


We would like to take a moment and dispel misconceptions of the word "Nodal Point". When referring to this no parallax point (NPP), many still refer to it as the Nodal Point. Technically, the point at which you would rotate the camera/lens to avoid parallax is called the "entrance pupil". The entrance pupil is a floating point located inside the lens, at the point where the light refracts or reverses itself before continuing to the image sensor or film plane. The entrance pupil is different on each lens and changes at different focal lengths.
Nikon
The entrance pupil (incorrectly referred to by some as nodal point) of a lens is the virtual image of the aperture formed by the lens elements in front of it, and is the place where light paths cross before being focused onto the CMOS, CCD or film plane. For a light ray to pass through the lens, it must be directed towards the entrance pupil, which is therefore the center of perspective. Note that the entrance pupil can be outside of the lens and even behind the film plane! When taking adjacent images you want to rotate the camera around a line that runs through (or very close to) the entrance pupil. By finding the entrance pupil of the lens, and rotating the camera about this point, you will assure parallax free images. Parallax is more obvious in the viewfinder with objects close to the camera. The more distant the objects, the lesser the parallax. If using a lens with zoom capability, the entrance pupil will change in position as the zoom changes. Because of so many variables, it is best to learn how to find the entrance pupil manually. But if you want to see if we have your settings please look on our website ( www.nodalninja.com ).

To locate the NNP of a lens, have the camera/lens setup on Nodal Ninja as shown below. You want the camera as far back on the upper rail as possible, while the lens is pointing towards the horizon and parallel with the ground.

Nikon camera on Nodal Ninja 3
Position in front of the camera two objects - one nearer than the other. For example, indoors you could use two lamps, one at about 6 feet with the other at about 10 feet, or if outside try using two fence posts or light poles. Any two objects creating a vertical line will work as long as one is positioned behind the other. In this example we'll use two floor lamps.

Finding the Entrance Pupil of a lens
With the camera mounted in portrait mode (sideways on the upper rail) and looking inside the viewfinder, or at the LCD screen, position the camera so the two objects are towards the left side of the frame, as shown. Note the exact position of the two lamps. Ideally place the lamps so they aren't directly behind each other, causing a slight offset.

Now rotate the camera so the two lamps are now in the right side of the viewfinder frame. Go back and forth a few times, and watch to see if the apparent distance between the two lamps changes. If the apparent distance changes, like in our example, even slightly, then you have parallax.

Finding the Entrance Pupil of a lens
You'll now need to loosen the camera mounting knob enough to allow you to move the camera slightly forward on the upper rail - about 2mm. The marks on the rail systems are in mm's, so just move to tick marks forward. Repeat above steps. Keep doing this until you reach the point where you notice no movement between the lamps.

The objective is to move / be able to rotate the camera left to right without seeing any apparent movement of the lamps, as shown below.

Finding the Entrance Pupil of a lens
Finding the Entrance Pupil of a lens
Note the position on both upper and lower rails and use the rail stops to set the position. If you change cameras or lenses, you'll need to repeat the above steps.
Now you're ready to start shooting panoramas.

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