by the way, widest (fisheye) lens for Canon FF is: EF 8-15mm/f4L (180 diagonal FOV). doesn't take front filters.
a more affordable option would be the sigma 8-16 (doesn't take front filters), or for those that take front filters, can consider: 10-22 and tokina 11-16/2.8. anything wider, you can do a panorama & stitch.
you can correct the distortions during PP with software, but you will lose part of the image.