Experiment with a flat simple mirror

Take a picture of the fisheye lens by itself using a mirror.

Put the camera on a mirror as shown on this picture and hold it firmly to keep the bottom of the body flat on the mirror.

Now take a photograph in circular fisheye format :

By taking a picture of reflected image of the fisheye lens on a mirror, you shall be able to

(*)The roll error is the angle between the bottom plane of the camera body (which is generally attached on the top of a pano head), and the "horizontal" axis of the CCD sensor of the CP990. It should be null as the CCD is supposed to be horizontally aligned.

This error gives headache to those who cannot figure out how to improve the adjustment of their panhead after trying everything : Optimizer from PTools shows this systematic error in the resulting script !!!