I particularly like working with rectangular fisheye lenses for my wide-angle macro work because they exhibit better good close-focusing capability than normal ultra-wide lenses. I have extolled the virtues of  the Sigma 15 mm f/2.8  rectangular fisheye on many occasions when I was using it on APS-C format (with Nikon bodies and with a Sony NEX 7). Using a lens like this, designed for full frame, with formats smaller than that, permits the utilisation of the centre part of the image circle. The images then show remarkably little distortion – just a bit of curvature…