I have a lens that gives perfectly decent results at f/22 and f/32. Here's its capture of an ISO 12233 test chart:
It's not the sharpest lens I own, but whaddaya want, it's a quarter century old design. (SMC Pentax-A 135mm/2.8)
This lens starts getting soft at f/16 and below. Between about f/11 and f/5.6, it's reasonably sharp in the center, with an increasing amount of that "vaseline on the skylight filter" look at the edges. By f/4, the soft focus effect reaches the center, but it's not so bad that you couldn't purposely use it as a soft-focus lens. At f/2.8, it's an, um, "creative" dreamy lens. Yeah, that's it, dreamy:
Note the doubled image of the triangles at the edge of the test chart.
What does this test result mean?
I've asked two lens repair companies to quote me on a repair for this. One said "talk to Pentax," and the other didn't even bother responding.
I ended up buying a different copy, which performs properly through the full aperture range.
That leaves me with a set of choices:
Sell it, including these test chart images of course, hoping that someone wants a "creative" lens.
Keep it and use it as such myself. But, I have Photoshop, so what do I need with a soft-focus lens?
Take it apart and try to fix it. If it doesn't work out, hey, I get a pile of bare lens elements to play with. That could be fun.
There is no visible problem when looking through the bare lens, and it rattles only a tiny bit when you shake it hard. I'm guessing the tiny rattle is a loose lens element. Does the test image support that guess?