The Leica 50mm f/2 APO-Summicron-M ASPH is arguably Leica’s flagship lens. So how well corrected is it for color? Really well, and as good as other Leica M APO lenses, but that is a relative statement of course.
The visible spectrum does not focus in the same plane unless the lens is something really special.
An APO (apochromatic) lens makes some (undefined) extra optical effort to reduce the disparity in the plane of focus across the visible light spectrum (primary longitudinal chromatic aberration aka LOCA and secondary longitudinal chromatic aberration) as well as correcting color fringes (lateral chromatic aberration).
But shoot with colored filters on a monochrome high-resolution digital camera such as the Leica M Monochrom: this is unforgiving of errors. It quickly becomes clear just how well corrected a lens is (or isn’t) for longitudinal chromatic aberration by the simple result of focus accuracy.
Filters assessed and compared to unfiltered include blue, green, yellow, light orange, orange, red, dark red.