Tuesday, March 23, 2010

HDR Aquisition - Pano Head Setup

Before we begin.  Sorry this post has taken so long to finish.  Life gets in the way!  Enjoy!

Part of the art and science of taking good panoramic imagery is stitching together multiple images with as little parallax shift as possible.  This means you need to align the nodal point/entrance pupil around the center of your rotation axis of the panoramic head.

Most everyone I know refers to this "no-parallax" point of the lens as the "nodal point" but while writing this entry I have found that this is actually called the Entrance Pupil.   Here's a little bit of a cut and paste from that wikipedia entry.
The geometric location of the entrance pupil is the vertex of the camera's angle of view[1] and consequently its center of perspective, perspective point, view point, projection centre[2] or no-parallax point[3]. When the optical system is physically rotated about its entrance pupil, the perspective geometry of its image does not change. In panoramic photography, for example, it is important to rotate or pivot the camera about its entrance pupil in order to avoid parallax errors in the final, stitched panorama[4][5]. Depending on the lens design, the entrance pupil location on the optical axis may be behind, within or in front of the lens system; and even at infinite distance from the lens in the case of telecentric systems.
Ok now you know what then entrance pupil is what next.  Lets calibrate our Nodal Ninja.  The Nodal Ninja like most panoramic rigs operate in a similar manner.  You have to align your camera in 2 different axis in order to have your camera in the proper position on the pano head.
A. Align the center of your lens with the center rotation axis of your panoramic head.

We'll call this adjustment the left-right adjustment.  First mount the camera on the Horizontal arm.  Rotate the horizontal arm until the camera is pointed directly down at the center rotational point of the pano head.  Now adjust the "left-right" of the vertical arm until the lens is directly over the center rotational point.  It is sometimes easier to put a longer lens on the camera so you can see the center of the rig a big better.  Once you have this properly adjusted for your camera this setting will never change no matter what lens you have on the camera.

If this isn't correctly aligned you will get a broken image at the nadir point of a stitched panoramic.

John Houghton explains this very well.
An indication that the lateral position is not quite right is a broken, sawtooth edge to the head at the nadir in a stitched panorama (an unpatched nadir, of course).  The example on the right is a typical example:
The top of the head looks somewhat like a circular saw. In this case, the "teeth" are set for cutting with a clockwise rotation of the saw.  This indicates that the entrance pupil is offset to the left of the pano head axis, as viewed from the back of the camera, so the camera needs to be shifted a little to the right. If the saw is set for cutting with a counter-clockwise rotation, then the camera needs to be shifted to the left. (image to the right is from John's site) 

B. Align the entrance pupil of your lens with the center rotation axis of your panoramic head.
Next we will align the front back position of the camera on the horizontal arm.  A simple way to do this is to line up two vertical items.  Place one close to camera and the second item further way.  From a center framed position these two items should appear to be stacked on top of each other.  Rotate the camera to the left and right about 20-30 degrees.  When rotated the two items will be in exactly the same position if the camera is aligned.  If the two items do not appear to be "stacked" on top of each other, then simply adjust the forward or back position camera until the items are aligned.

Fisheye Considerations
John Houghton also notes that you need to be aware the entrance pupil for a fisheye is not centered on a single point.  Basically he says that you want to adjust your entrance pupil to correspond with the angle your stitch.  He also notes that this will compromise the accuracy of your zenith and nadir points, but stitching software is usually good enough to fix it.  Check out his site for the full explanation.

Now your probably wondering why bother with both axis when shooting with a fisheye?  You could easily use a simpler rig like the Nodal Ninja 180.  With the NN180 you only align the entrance pupil on one axis and not two.  Wouldn't this be easier on set?

This is true.  But using a panoramic head designed specifically a fisheye limits you to just using a fisheye.  The NN5 allows you do use any lens.  And depending on the lens used you get a significantly higher resolution final panoramic than one created using a fisheye lens.  If you need to capture a high resolution sky or cityscape your in trouble if you only have a fisheye rig.  End of the day if you have the extra coin to buy multiple rigs... go for it!  They happily sell you more than one, but for me one rig that can do both is a bit more cost effective.

Taking your time at this step will result in panoramas that take minutes to stitch together.  And the team back at the office will love you for it!  In fact they may pick you up on their shoulders and parade you around the office, like the hero that you are!

Next time, lets shoot some pictures!

Here is a quick resource list that I came across in my research for this entry.  These guys have put together some very good sites which inspired me while writing this blog entry.

Michel Toby's website
johnpanos.com - Finding the No-Parallax Point
The Nodal Point (This has a very cool laser plotted graph of a sigma 8mm lens's entrance pupil)
Nodal Ninja website
Digital Grin (nodal point tutorial)
The Really Right Stuff
Big Ben's Panoramic Tutorials 
The "Grid" tutorial

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