Samyang, Rokinon, Vivitar, Bell, and Howell are identical lenses but for the name on the barrel. all they do is repackage it differently with the same optics.
We are going to compare the perspective difference between the Vivitar 7mm f3.5 Fisheye and the Canon 15mm f2.8 Fisheye. We will test both lenses on the Crop sensor Canon 7D first. then test it on the full frame Canon 5d Mark II. Then take a closer look. For the test the tripod stayed in the same spot.
