This one's probably for equipment nerds only.  The widest-angle lens I own for my Canon 5D Mark III bodies is a 17-40mm f/4 L, which I use only occasionally when I need to capture something (but not usually someone) that my 24-70mm f/2.8 L (II) is too long for, but I've found myself in situations when even 17mm wasn't quite wide enough.  In February of this year, Canon announced the 11-24mm f/4 L, which is the widest-angle rectilinear zoom lens in the world.  It's only recently become available in UK shops and yesterday I tried one out to see for myself how much more another six degrees of focal length represented through the viewfinder.  The middle of the photo below is what the lens offers at 17mm, compared to the whole frame at 11mm, so the area in green shows that this new lens can see around 50% more.  However, the glass required to do this protrudes beyond the front of the lens so there's no scope for a protective filter, its weight is more than twice that of my 17-40mm and it costs over five times more.  Please form an orderly queue behind me.