You mentioned "I want to purchase the Fisheylens 4.5mm F2.8 EX DC HSM Circular Fisheye"
You may or may not get a full circle image on your camera. Canon 5d, 5dm2 and 5dm2 are all use a full 35mm frame sensor 24mm x 36mm.
IF your camera is a full 35mm frame sensor [_].. then yes the full circle "O" should be inside the frame borders of your full frame sensor [O].
IF your camera is not a full frame sensor.. then you probably have a Crop factor. ([_]) meaning the full circle image exceeds the width of your sensor.
Typically Canon cameras have a crop factor of 1.6 meaning if your lens is an 8mm then 8 x 1.6 = 12.8 mm effective 35mm
Typically Nikon cameras have a crop factor of 1.5 meaning 8mm x 1.5 = 12 mm effective 35mm
Good reference is
http://en.wikipedia.org/wiki/Crop_factor
We use a Nodal Ninja R1 panorama head with a Sigma 8mm full circle Fisheye on a Canon 5d full frame sensor, tilt up only 5° to achieve zenith (top) closure, 4 images around horizon. No zenith [up] or nadir [down] required.
We also use a Nodal Ninja R1 pano head with a Sigma 8mm on a Canon T1i (NOT full frame sensor) tilt up only 5° to achieve zenith (top) closure, 4 images around horizon. No zenith [up] or nadir [down] required.
We achieve covering the nadir [down] hole using a tripod cap picture with advertising clients name or our company name. OR after stitching we use Pano2vr and just limit the down angle so folks can not see the hole.
The Holbox Island Hotel pano was taken using the Canon T1i [500d] and Sigma 8mm fisheye, 4 images around with a tripod cap 5° tilt up on a handheld monopod.
http://360texas.com/services/sr/dbu/index.htm
Hope this helps
