Scott M comments on the Leica 50mm f/2 APO-Summicron-M ASPH:
After it was announced I started to think about this lens - why was it needed? Then I finally thought of a possibility, triggered by the simultaneous release of the Leica M Monochrom.
With a color camera, you can correct lateral and now longitudinal chromatic aberration in post (or in-camera; thankfully it's now a pretty trivial procedure even with OEM raw converters like DPP) with only positive effects on image quality... *because you have color information to calculate the correction/s required*. You can probably see where I'm going with this... :)
But with a true monochrome sensor, you have no such CA data "embedded" in the image and therefore be unable to make the best potential of the missing Bayer CFA without a "perfect" lens. It may even look *worse* due to uncorrectable color errors that just look like smear/blur in a monochrome image. So, the new lens may have been - in a round-about way - necessary to squeeze every last lp/mm out of the new camera (which I'm sure it does - for the price, it had better!)
Thanks again for a very imformative site - I was especially interested in your take on the Olympus E-M5 which while not on my short-list either is turning out to be a real benchmark in the mirrorless segment (I recently bought a G2 on sale for $299 and it has been a surprising amount of fun to use as a not-quite-compact)
DIGLOYD: The supposition is on target. But I think the new lens also is consistent with Leica steadily pushing the state of the art forward.
By using a colored filter, one can eliminate color smearing especially in out of focus areas. See Filtration for Leica M Monochrom to Improve Image Sharpness and Tonal Separation. See also my filters page, with handy links for 39/46/60mm filters for black and white photography.
It is not always desirable to filter, but it can be highly effective, because it simply eliminates part of the spectrum. Even the new 50/2 APO ASPH will probably have some out of focus color effects (secondary longitudinal chromatic aberration), since all the Leica M APO lenses show such effects. We shall see. Still, a well corrected lens is alway the best way to make images, rather than fixing flaws in RAW conversion or via post-processing.
The catch: many newer Leica M lenses are a hassle to use with filters, e.g., the newer lenses with the rectangular integral lens hoods, which won’t allow a standard filter to mount.