Circular Fisheye

The Sigma lens has a two part lens cap. you must remove both parts to get full 180 degree coverage. You can also use the second part to your advantage. Block off unwanted parts, but if the light is shinning the wrong way it will show in the final image (d1). See pictures a1, a2, d1 and e1 as examples where the second part of the cap was not removed. There are also 3 pictures that show a crop of actual pixels.