How to use a fisheye lens on a Canon T3i compared with a T2i?

Question

The question may seem simple, so let me explain it more:

I had a Canon T2i, which I used for a time with my fish eye lens. When zoomed all the way out on the fisheye lens there used to be visible in the corners black space (the fish eye image is circular and so the sensor would capture the areas outside the circle as black space). This effect was desired.

Now I am using a Canon T3i with the same lens, but on this camera when the lens is zoomed all the way out there is no area outside of the circle visible in my photos, as if the sensor is closer to the lens or something and simply can't achieve the same level of zoomed-outness. The entire circular image fits within the frame, there is no negative space captured as with the T2i.

This seems a little odd to me, I assumed these two models were relatively similar but this difference seems pretty dramatic to me, and I don't have a good explanation as to why it is this way nor do I know if there is anyway I can correct it. Is there a way I can use the T3i with the fisheye achieving the same effect as the fisheye on the T2i? Do I need to buy a real full-frame camera?

Answer 1

Wow, answered my own question:

There is a switch on the fisheye lens labeled "LIMIT" which will lock the maximum zoomed out state of the lens to a point where no negative space is captured on non-full-frame sensors. When this LIMIT switch is in the non-active position, the lens does not impose any artificial limits and the you are free to capture a circular image.

I guess I never knew what this switch did and in the process of switching lenses in and out I must have accidentally activated the limit....sorry for the dud question!