I used to own the 15mm, liked it a lot, used for professional work a couple of times - to my full satisfaction - very good in direct sunlight! Being a true GASer, I ffound some reason to sell it. Later I have bought the 12 and tried it out a bit on a trip to Mexico. vignetting, yes, but else; rather impressive. (Am going to scan a few from that trip this evening). The 15 is however handier, smaller - as is the 15 viewfinder. I could actually imagine having them borh ...