Acratech Spherical Panoramic Head for Sigma 8mm f/3.5 Fisheye LensesThe Acratech Spherical Panoramic Head is a special use head designed for use with the Sigma 8mm f3. 5 EX DG circular fisheye lens which captures a full 180 field of view. The head allows precision capture of panoramic images with indents at each 90 turn. The engraved markings aid in the proper positioning of the camera. Software is necessary to stitch the four images together. All knobs are secured, so that they will not fall off if they become too