A super wide angle lens is a handy thing. The ZF.2/ZE Zeiss 15mm f/2.8 Distagon make an excellent complement to the Zeiss 21m f/2.8 Distagon.
The 15/2.8 Distagon is suitable for Nikon or Canon or (with lens adapter) Sony mirrorless or on Leica M240.
Images up to 24 megapixels, including crops. All examples were shot with the 50-megapixel Canon 5DS R using the ZF.2 model (with lens adapter).