elliott937 wrote:
You have just given me an idea. When I use my Sigma 8mm with my 5Dii, I to capture a very large circle. However, since you said you have a less-than-full frame, you capture a rectangle image. So, thank to you, I've pulled out my retired Canon 20D body, which I'm sure is less than full frame, and perhaps the 8mm will capture a rectangular image on the 20D. I've put batteries on the charge first, so that I can power up the 20D. I'm keeping my fingers crossed.
That should give you the rectangular image you're looking for. It will also work if you crop the circular image that your 5D produces.