Unveil the roll error of the NIKON CoolPix
When using the fisheye, the photographer can readily observe the offset position of the circular image in the rectangular frame of the CCD ( Mode Fisheye1 or fully zoomed in manual mode). This yaw ( or pitch in landscape mode) error is thus observable and can be corrected.
A roll error (rotation around the optical axis of the lens) it is much more difficult to observe. In fact this error is often not perceived at all. The photographer will most often think afterwards that his camera, its panohead, or both, were not horizontally level during that shooting session!
There is a way to observe this elusive roll error of the Coolpix camera :
From the original circular fisheye image above, see below the zoomed extract of the bottom. It shows the front of the fisheye lens distorted by the fisheye effect. It looks almost perfectly flat !
As the camera BODY is put flat on the mirror, the two opposite sides of the lens front ring should both be at the same level on an horizontal line. They are not (the green line is horizontal ). The cant angle can be estimated at -1.8deg and is a roll angle error. The CCD sensor of the CP990 is actually mounted in the camera with such an error with respect to the bottom mounting plane of the BODY. An "action" could easily be scripted in photoshop to counteract this intrinsic error.