Stitcher is designed to stitch images captured with both rectilinear camera lenses and "Fish-eye" lenses. "Fish-eye" images are shots taken with an extremely wide-angle lens, as much as 180 degrees, and have considerable barrel distortion. The straight lines at the edges of a panorama appear to curve around the center of the image.
"Fish-eye" lenses are used to create wide-angle perspectively corrected image. "Fish-eye" lenses are designed to capture a very wide field of view. A wide field of view reduces the total number of images you must take to complete the panorama, allowing you to shoot a panorama faster and easier.
Normally if you have selected Read EXIF data in the Preferences dialog, Stitcher® will try to automatically calibrate your "Fish-eye" lens. But in some cases, for example, when the exif data are missing or when the images
do not contain enough information, you will need to calibrate your "Fish-eye" lens manually.
The circle allows you to crop the image in order to delete unnecessary information.
