I want to take 360º panoramas with my 5D. Can I use the sigma 15mm rectangular fisheye or will I need a circular fisheye?
If it can be done with the sigma 15mm rectangular fisheye, how many shots wilI I need?
You can make a 360 pano with any lens, but the wider it is the easier. I am guessing you are talking about a 15mm fisheye.
The sigma 15mm produces a image circle centered on the image plane and the edges at the left and right on a full frame sensor. The top and bottom are cut off. To create a 360, the easiest is the rotate the camera into portrait mode, and take 6 images in a circle. If you have a crop sensor camera, the image will produce about a 180 fov across the diagonal, so it will take more photo to cover the full visual field.
I decided on the Canon 8-15mm F/4 zoom for my 360 Panos, which by the way is may favorite lens.