My enthusiastic impressions of the Leica 28mm f/2 APO-Summmicron-SL ASPH in APS-C crop mode are tempered significantly by the flagged-as-mandatory distortion correction, which cuts down MTF over most of the frame to make the Leica MTF charts an exercise in deception. Leica can design a lens however they want, but they ought to at least be honest in their marketing materials.
Includes corrected vs uncorrected example image.
UPDATE: I added another example; it looks even worse.
Leica must have a 'thing' for 28mm optical design compromises, because the Leica Q2/Q have horrific barrel distortion. This situation is not nearly so bad, but it is enough that it cannot go uncorrected for many images.
Don’t read me wrong here—the 28/2 APO-SL is a fantastic lens, and all lenses have compromises. But it’s just the decision to compromise sharpness and not be honest about it that bugs me both in those terms and in practical ones (across the frame sharpness for landscape), yet the 3D pop in central areas, the superb correction for secondary color and aberrations are really terrific.
There is a reason that Leica omits distortion from the datasheet for the 28/2 SL: they don’t want you to know, and flagging distortion correction as required lets them hide it. Leica M lens data sheet always included distortion (and vignetting) charts. See for example the datasheet for the Leica 28mm f/2 Summicron-M ASPH, or any other M lens. OK, that was a bit of mind reading, but clearly Leica is not keen on revealing the true distortion of the SL lenses, or vignetting for that matter. When you have Peter Karbe waxing on the fantastic resolving power of the SL lenses while failing to mention distortion correction—I call that lying by omission.