The Samyang 12mm F2.8 ED AS NC Fish-eye ($599) is one of the widest lenses you'll find that works with a full-frame camera , but despite this fact - as well as its f/2.8 aperture - it's quite light and compact.
Full-frame coverage; Wide aperture; Compact; Ultra-wide field of view; Close focus capability; Available for multiple camera systems
Fish-eye look isn't for everyone; Manual focus only