How to shoot a Nadir image for a spherical panorama with a rectilinear lens?

Question

Trying to shoot a complete spherical panorama, I found that no software was able to stitch correctly the nadir images I had. PTGui Pro 9.1 and Autopano Giga came close but no cigar as they say. I found a vague mention that says is is possible in an answer to this question which says:

There is of course a way to shoot nadir and zenith with a tripod to eliminate it from the picture but believe me it is extremely complicated and involves moving the whole tripod several times.

This brings me back to my question:

How to shoot a Nadir image for a spherical panorama with a rectilinear lens?

For one, I do not have a fisheye lens. Plus, this severely limits the resolution of resulting panoramas, so I really want a solution that works with any rectilinear lens, although most importantly between 12 and 55mm on an APS-C sensor or 16 and 75 on a full-frame, since those are the most common focal-lengths I have with cameras of the respective sensor sizes.

Answer 1

There's a tutorial on Panoramas.dk where the basic idea seems to be using a hanging locked measuring tape from camera to spot marked on ground with a lens cap to make sure you can hang the camera on a slanted support so that its nodal point is in exactly the same place where it was on tripod; you also have to watch out for shadows created by your support (so pick a suitable angle).

Answer 2

See: John Houghton's viewpoint tutorial.

The highlights version:

If you're using PTGui, and you're shooting in good light that allows for handheld, then just remove the tripod and handhold the shot. You can even shoot it slightly obliquely from the side (to avoid shadows) if you have to.

Don't include this handheld nadir in the primary stitching project. Only bring it in when all your on-tripod shots have been aligned properly. Add the handheld shot, define some control points, mask out the edges (i.e., try to keep it to a flat area that will patch out the tripod head), mask out your tripod and panohead in the on-head nadir shots (most folks typically take two of them, rotated 180 degrees to remove the vertical arm of the head from the shot).

Then, when you go into the optimizer, make sure the advanced stuff is showing, and clear the checkboxes for all the other images (i.e., locking them in place), and only check the yaw/pitch/roll boxes to position the handheld patch. Run the optimizer, get really lousy results. Don't panic. You're not done yet.

Now set that magic fourth checkbox for "viewpoint". PTGui then adjusts the plane of the patch to best match the control points. When you run the optimizer this time, your errors should be acceptable.

Then stitch.