The Sigma 20mm f/1.8 EX DG is a fast-aperture wide-angle prime for Canon full-frame cameras. Key features include:

Fast and Wide. If you’re looking to shoot sweeping cityscapes at night, the Sigma 20mm f/1.8 has you covered. Its 94.5º field of view captures a very wide view, while its f/1.8 aperture lets in more light than most other wide-angle options.

Focusing. The lens can focus as close as 7 inches (just 2.6 inches from front of lens to subject), which allows for some interesting “wide-angle pseudo-macro” shots. Its rear-focusing design ensures both filter compatibility and consistent physical size. A bit of an odd note, this lens has both a “clutch” type switch (utilized by sliding the focus ring forward or backward) and a normal AF/MF switch for turning Auto Focus on and off.  

Creatively Flawed. Sigma markets it as excellent for landscape photographers because of its excellent optical performance, but it’s better known as wide-angle lens for emphasizing center subjects. We’re not saying it’s bad; just be aware of its less-than-sharp corner performance.