one thing about the 21 that is the kind of lens where after purchase you feel like its "never sell " piece of glass.
maybe its the steel construction, the coatings are first rate and i have read technical discussions written by Zeiss optical engineers, where they really touched on the importance of the coatings -to properly correct to bring the lens close to being apo quality.
I guess the 135 is the only true "apo" but the other lens are close.
so in other words even tho its a landscape lens for the most part it still will do crazy accurate floral images which have "pop" kind of like the Nikon 200 F4 micro. (another fantastic lens)
one thing with the 21 is that many folks dont know how to use it.
their images dont look right. the lens really wants to have a nice foreground in the composition.