We use and industry standard ISO test chart to measure resolution, shooting the chart at each major ISO increment and testing both raw and JPEG files.
We've compared the performance of the Leica SL against three main rivals:
Sony A7 II: This camera offers the same resolution sensor as the Leica SL but costs a good deal less.
Sony A7R II: The highest resolution Sony A7-series camera with 42.4 megapixels and 4K video capability.
Nikon D4S: Until we can get a Nikon D5 in for review, this is the top DSLR rival. The D4S has a lower resolution, however, and shoots 1080 video not 4K.
JPEG resolution analysis: Not surprisingly, the Sony A7R II is way ahead of the rest for resolution, but the Leica SL is in second place. What's especially striking is the way it holds on to its resolution right the way through into the upper ISO ranges, easily beating the Nikon D4S and stretching away from the Sony A7 II at ISO 800 and beyond.
Raw resolution analysis: The result for raw files are closer. The 42.4MP Sony A7R II still has an advantage, but it's much reduced and the Leica SL is not too far behind in second place, catching up with the A7R II at ISO 12,800.