Get Leica 28mm f/2.8 Elmarit-M ASPH at B&H Photo.
See also the evaluation with the Leica v2016 28mm f/2 Summicron-M ASPH. The two studies together also provide additional insight between the lenses, such as performance at f/2.8 between the lenses.
One of the first things I check for with any new lens is focus shift, because it’s so critical a factor. This assessment is at minimum object distance (MOD) with the 2016 version of the Leica 28mm f/2.8 Elmarit-M ASPH: