At least used on APS-c a lot of those soft edges are cropped off. People demanding FF coverage for their older Pentax glass, should be careful what they wish for. That being said, my 18-135 theoretically has terribly soft edges, describes as coke bottle bottoms, yet of the 20,000 images taken with this lens, the apparently soft borders was an issue in maybe 2 or 3 images. People forget, the way Pentax designed lenses was a design philosophy. "Lenses designed for the way people take pictures, not to do well on numeric charts." And while we can't expect everyone creeping into Pentax land to understand the merits of that design philosophy, Pentax lens designers certainly did. I just find it odd, that folks now attribute the Pentax look to some kind of trade off, an un-intentional side effect of the desire for lighter smaller lenses. On the same site where people have to switch to Nikon or Canon to get better rendering in their out of focus areas. Pentax has always said, they didn't deign for test charts, although that changed with the DA* 60-250.
It's somewhat ironic. That so many buy into Pentax for the price and old glass, that so many didn't realize was designed with a different design philosophy, and they squawk like a bunch of chickens when they find out Pentax was never just a Canikon clone. But Pentax FA glass, was designed to have a centre as sharp as any available and edges that gave smooth bokeh, because it was a bit softer. Those who take pictures in a style for which it was intended love it. For those technical lenses, appropriate to architecture etc. may have to look somewhere else, like Sigma or Tamron for their lenses. Or just switch to one of the companies that have designed for even performance on a test chart as part of their design philosophy for years.
Just with my experience with the "horribly soft" 18-135 @ 135mm, I can see what the Pentax engineers understood. If Pentax traded centre sharpness for edge sharpness in this image… it certainly worked for me.
I don't pretend to know all the ins and outs of Pentax lens design. But I know there was a design philosophy that apparently cost a lot of Pentax engineers their jobs, when Hoya wanted to make them Canon, Nikon, Sigma clones. And that as one who has some edge sharp glass that performs well on the test charts, the world will never see glass like the old FA glass again. The design team chose not to change philosophy to the Canikon model and were fired.
Pentax engineers were clearly looking at other factors besides the measuables.. distortion, CA and edge sharpness. I think they were looking at how those effects could be combined to produce an image that would be pleasing for 90% of what most photographer took pictures of. Exactly what they were looking at we'll probably never know. But I suspect they were looking at things like, how pleasing are the images rather than how sharp is the glass.
I have a Sigma 8-16, sharp edge to edge, distortion corrected etc. but my wife hates the picture taken with it. If she's not right dead centre, they actually distort how she looks. She looks much more natural with an uncorrected lens. Your eye is not used to seeing uncorrected images. They don't look natural. The human eye is one lens, and the brain is used to seeing a certain amount of field curvature and softness at the edges. Only in this day and age of technology that ignores human form would you even have to make this statement.
SO now you get people complaining about soft edges, as if there is something wrong with that. Use the glass for what it was designed for, and you won't have issues with soft edges. Especially, if you just throw out the numbers and start shooting. Because their are qualities in these images, my guess is, you aren't ever going to see again, in new lenses.
And I'm going to say right up front. If you don't appreciate those qualities, you shouldn't be a Pentax shooter. The goal was never to be Canikon clones, especially in the area of lens design., but to be a unique company with a different philosophy. It would seem, a lot of people just don't get that, and continue to harp on the "center sharp, edges not so sharp thing." There is a lot more to a lens than sharpness. It's unfortunate that so many come with the attitude that equal sharpness across the frame is a goal worth attaining, per se. Maybe, maybe not. I fail to see why a style of lens that is more desirable shooting buildings with straight lines on one plane should be considered the best type of lens for everyone.
The old Pentax guys knew they weren't. That's the Pentax "thing". Those who came to Pentax with knowledge of the product, always knew that. Leaving it to for those who have most of their experience with other systems to say "hey something's different". Yes something is different. But that doesn't mean the old users want it changed. And why the new comers would want every lens to be a heavily corrected -sharp edge to edge, technical marvel, that doesn't take realistic looking images , I have no idea. It's part of the "everyone has to be the same" philosophy that got the Pentax design team fired. If you're taking pictures of buildings with straight lines, use the Sigma 8-16, if you're taking pictures of people use the 10-17 fisheye. You'll have a lot more fun that way. And don't let anyone convince you you only need the Sigma. Its a technically better lens, but it doesn't appeal to the human aesthetic when used for images of people.
And if you really think the old Pentax design team was wrong… there are lot's of other companies that think the way you think, enjoy them.. Too bad you can't enjoy the Pentax guys, but, there's absolutely no pay off in wishing Pentax was the same as all the others. They weren't. Intentionally. It's not that they were wrong, it's that they were different.
After paying a lot for a heavily corrected lens with low CA and next to no distortion, and looking at the images, all I can say is, "you folks are being conned." As a walk around lens with people in the frame, it's an inferior lens. Excelling on the tech specs on the test charts does not make good people lenses.