Tim Gray
Well-known
APO correction makes B&W images sharper. Same reason one might use APO enlarging lenses for B&W printing.
It’s just more of the same reason most non-APO lenses aren’t simple lenses, but achromats. If it didn’t matter for B&W, we’d all be shooting with single element simple lenses. That works okay if the source scene is (or recording medium is sensitive to) a single color (narrow band of wavelenths), but that isn’t B&W photography. Our recording medium might be monochromatic, but it is sensitive to many wavelengths of light which are out there in the world we photograph.
It’s just more of the same reason most non-APO lenses aren’t simple lenses, but achromats. If it didn’t matter for B&W, we’d all be shooting with single element simple lenses. That works okay if the source scene is (or recording medium is sensitive to) a single color (narrow band of wavelenths), but that isn’t B&W photography. Our recording medium might be monochromatic, but it is sensitive to many wavelengths of light which are out there in the world we photograph.