Gear Guide FISHEYE FRAMING Very short-focal-length lenses encompass a full 180 degrees diagonally, but with a curved horizon. These fisheye lenses (a TTArtisan 11-mm f/2.8 was used at left) are specialized lenses ideal for wide sweeps of the Milky Way, aurorae, or, in this case, both at once. 11-mm full-frame fish-eye lens RECTILINEAR DISTORTION Most lenses 14-mm and longer are rectilinear designs, which present straight horizons but introduce a leaningin distortion on vertical structures. The effect is more pronounced on wider lenses. This image was taken with a Rokinon 14-mm SP at f/2.4. 14-mm rectilinear lens ADAPTED ABERRATIONS At far left is a blowup of the frame corner from a 14-mm Sigma Art lens at f/1.8 mounted directly to a Nikon D750. At center is the same lens on a Sony a7III using a Commlite adapter and at near left with a Metabones adapter. The slightly incorrect lens spacing introduced by some third-party adapters increases off-axis aberrations. Sigma 14-mm " Native " on Nikon D750 Sigma 14-mm on Sony α7III via Commlite adapter 32 JANUARY 2022 * SKY & TELESCOPE Sigma 14-mm on Sony α7III via Metabones adapter