Originally Posted by Jos Segers
The AL in this lens will prove its worth when you are shooting at minimum aperture of f22; in a non-corrected lens, diffraction will introduce softness into the image caused by light bending past then refocusing onto the common point. In all lenses, blue and green wavelengths can be brought to a common point fairly easily, but not red. With AL elements, one or more concentrate all three wavelengths to the one point, resulting in much improved colour and definition. This is why you should not shoot at f22 or f32 with a 55 or 45mm lens! Most of the SMC Pentax (modern) lenses are well corrected for their purpose; the 75mm AL must obviously have been Pentax's Eureka moment in lens design, much more so than its 75mm shift lens.
I would be interested in seeing MTF charts of this and other lens characteristics by Pentax, but they are very scant in terms of web availability. Only hobbyists seem to have taken any effort to test them. The best evidence of performance is to speak to people who use the lens.