[Nass]

That accords with what I've observed in my tests. I think the issue with reversed lenses is field curvature. I don't know why this is reduced by stacking lenses, but it certainly is, in my experience. But, again, you can still get excellent results without stacking. Unless you are photographing stamps or some other flat subject, corners don't usually matter.

Yep, my thoughts too. Nice that we came to same finding independently

Back to OP, there are really two grades of extension tubes for Pentax - the really shocking no aperture linkage $10 ones, and $100-$150 aperture linked ones made by Kenko, copied by some other brands I think. You do get what you paid for.

[kentster]

Thanks team! After serious scouring of the internet, I've gone ahead and purchased some metal tubes on Ebay for $50 with the option to return them. I was wary at first but after asking a few questions from the seller I think it'll be alright. I am only using them to reverse mount a 28mm lens so they don't need to be perfect. If I do get a macro lens it will be the fully manual Venus 60mm f2.8 macro from the hobbyist lens makers at Laowa. So hopefully I won't be needing any automatic tubes any time soon. If I do though I think it'll be a waiting game because after looking I've discovered the auto ones for k mount are pretty tough to find unless you want to pay top dollar from Japan.

IF you do get this...please report back with a review. I'm thinking on this lens as well.